Rail industry knowledge, experience and perceptions on the use of nature-based solutions as climate change adaptation measures in Australia and the United Kingdom

Nature-based solutions (NbS) have been identified as sustainable adaptation measures which could be applied to rail infrastructure in response to the impacts of climate change whilst also providing highly valued co-benefits. To date, however, only a limited number of examples of their use have been found in rail, and there has been little investigation into barriers to their uptake. We use an online questionnaire to examine rail industry professionals’ knowledge, experience and thoughts in relation to perceived and/or actual obstacles to the use of NbS as climate change adaptation (CCA) measures for railways, and establish what could aid their wider implementation. This research confirms multiple examples of NbS being used in rail which are not included in the literature, and identifies a lack of awareness of NbS as the largest perceived barrier to their uptake. Education on and promotion of NbS in the industry will therefore be key to its successful widespread deployment. Policy, standards, and client specification were viewed as the best vehicles to enable greater NbS uptake; rail NbS case studies are therefore recommended as means of gathering robust evidence and examples to inform the development of these instruments. Demonstration sites could be used to inform rail stakeholders and communities to garner wider support for the concept. These may also be valuable to the work of researchers and practitioners investigating the wider development and deployment of NbS as sustainable CCA measures across wider (non-rail) sectors and scenarios.


Introduction
Safe, reliable and affordable rail services support global economies, with railways providing a cleaner and more efficient means of moving freight to markets and people to employment and social amenities across countries and continents (Koks et al 2019, The World Bank 2022).Railway infrastructure is exposed and vulnerable to extreme weather (Lindgren et al 2009, Network Rail 2015, Koks et al 2019), and with rising global surface temperatures projected to further increase and worsen the frequency and scale of extreme weather events (IPCC 2021(IPCC , 2022b)), there is a growing need for the rail industry to adapt to the impacts of currently-faced weather events, and the conditions anticipated in future climate conditions (Davies et al 2014, Marteaux 2016).Notwithstanding a rapid growth in literature on climate change impacts on transport infrastructure and operations, research on climate threats and subsequent adaptation approaches for the rail industry is scant (OECD 2018, Wang et al 2020).The ability of railways to maintain operations during extreme weather conditions and recover from these quickly is crucial to ensure the continued provision of safe and dependable services (Koks et al 2019, Nolte n.d.).Climate change adaptation (CCA) is therefore a complex and urgent challenge in the management of rail infrastructure (Davies et al 2014), as confirmed by the United Nations Economic Commission for Europe Group of Experts on Climate Change Impacts and Adaptation for Transport Networks and Nodes (UNECE 2018).The Intergovernmental Panel on Climate Change (IPCC) defines adaptation as 'the process of adjustment to actual or expected climate and its effects, in order to moderate harm or exploit beneficial opportunities' (IPCC 2022a(IPCC , p 2898)).Three basic approaches to adaptation can be identified: Retreat (or avoid), Protect (providing a physical barrier to protect the infrastructure), or Accommodate (adapting the infrastructure itself) (Eichhorst 2009).These approaches are illustrated in figure 1 in the context of adaptation to sea level rise; however, in general they are applicable to all climate change impacts.
In addition to the three above approaches, adaptation to climate change can incorporate a variety of possible actions (Jones et al 2012).Alongside 'soft' management interventions such as early warning systems for extreme weather events, the predominant rail industry responses to climate-induced impacts involve 'grey' engineered solutions (Blackwood et al 2022), including concrete flood walls and overflow channels, for example.There has been increasing recognition that 'green' , nature-based solutions (NbS), such as green (vegetated) walls and natural drainage systems, can supplement grey measures (Seddon et al 2020), thus creating hybrid options which potentially provide optimal, more sustainable solutions, especially when co-benefits are considered (Fink 2016, Ruangpan et al 2020).For example, using vegetation alongside concrete drainage installations would bolster the CCA intervention's protection against and accommodation of anticipated increases in rainfall and flood events, and in addition, the NbS would improve water quality, help stabilise embankments, and provide a barrier to the dispersal of pollutants (Blackwood et al 2022).By directly addressing societal challenges such as disaster risk reduction and CCA, NbS solutions can intrinsically contribute to sustainable development (Cohen-Shacham et al 2019).Whilst humans have historically applied natural solutions in response to climatic variability (Jones et al 2012), the profile of NbS has increased significantly in recent years and the body of research conducted on its application in response to the impacts of climate change has grown rapidly (Seddon 2022).Despite considerable research having been conducted on NbS implementation in cities and urban areas (Frantzeskaki et al 2019), studies on its use on railways have remained scarce (Blackwood andRenaud 2022, Blackwood et al 2022).
A literature review found five examples of NbS in place on rail infrastructure globally, all of which involved the use of vegetated solutions in response to high precipitation causing flooding and/or an increased risk of erosion and landslides, along with multiple examples of NbS implemented in non-rail contexts which may be transferable to the rail industry (Blackwood et al 2022).For example, such NbS concepts include green walls, natural drainage solutions, reefs and mangroves, (see e.g.Cohen-Shacham et al 2016, Eisenberg and Polcher 2019).It is acknowledged that NbS remain a relatively new concept (albeit with a very rapid uptake) and, due to the potential lack of understanding over what NbS are (Sarabi et al 2019), further examples may be in place in the rail environment without being labelled as such (Blackwood et al 2022).The limited literature on NbS practices in rail, combined with the general scarcity of scientific data on rail CCA (OECD 2018, Wang et al 2020) may be due to the rail industry prioritising the implementation of immediate operational responses to extreme weather events rather than research longer-term solutions (Blackwood et al 2022); this may be exacerbated by rail organisations generally not employing in-house CCA experts, whilst climatologists and meteorologists are not railway experts (Quinn et al 2017).The identification of additional instances of NbS application in rail will therefore help promote the NbS concept and may encourage its wider uptake.The adoption of NbS on railway infrastructure relies upon the acceptance of the concept by rail industry stakeholders including those who design, construct, operate and maintain rail infrastructure; it is also dependent on these parties' implementation of climate change risk assessments (CCRAs) to identify the need for and to implement CCA of any form (soft/grey/green/hybrid). The likely barriers to NbS implementation by railways as found in the literature include safety concerns, land use constraints and stakeholder dependencies (Blackwood and Renaud 2022).Despite these challenges, due to railways' need to implement sustainable, long-term and cost-effective CCA solutions, investigation into the suitability of NbS application on rail infrastructure warrants further exploration, with Blackwood and Renaud (2022) recommending measures to support the development of the NbS concept in the rail environment.This research engaged with rail professionals to examine industry awareness on, examples of and attitudes towards the use of NbS as CCA measures for railway infrastructure, guided by the following research questions (RQ): RQ1: are there NbS being used as CCA measures in rail which are not included in the literature?RQ2: what are the challenges and barriers to the operationalisation of NbS as CCA measures for the rail industry?
RQ3: what could aid the operationalisation of NbS as CCA measures for the rail industry?An online survey was distributed to railway professionals to gauge their knowledge, experience and thoughts in relation to these issues.The questionnaire was also used to ascertain levels of rail industry CCRA practice and awareness levels on general CCA measures, to help establish whether processes currently used may lend themselves as vehicles to implement NbS more widely.A parallel mixed methods approach was applied to combine the collection and integration of both quantitative and qualitative data, whereby closed questions enabled the quantitative measurement and description of trends, attitudes and opinions, whilst qualitative data gained through open ended questions added richness to the survey results (Krosnick andPresser 2010, Creswell andCreswell 2018).Results are structured based on the above three RQs.As there has been extremely limited research conducted on the subject to date (Blackwood et al 2022), this study therefore gathers important new primary data and information which may inform and promote practical CCA options for the global rail industry to sustainably protect its infrastructure from, or adapt it to accommodate, the impacts of a changing climate.The outputs may also support the work of researchers and practitioners investigating the wider development and deployment of NbS and/or CCA measures across wider (non-rail) sectors and scenarios.

Methods
Between 10-31 May 2022, participants were provided with an anonymous link to a web-based survey which was administered using Jisc Online Surveys (Jisc 2023).The full questionnaire may be viewed in supplementary text S1.Purposive sampling was undertaken by targeting invitations and requests to participate to key stakeholder categories who are, or should be involved in CCRA for railway infrastructure (Infrastructure Sustainability Council 2021, Transport for New South Wales 2016).The intended audience for the questionnaire was confirmed via letter to gatekeepers in national (UK) and State/Territory (Australia) rail organisations, who were asked to distribute a link to the survey and invitation to participate within their respective corporations.The authors did not have visibility of the onward distribution of the invitation to participate.Contacts known to the lead author, who has over 15 years' experience in railway infrastructure sustainability in the UK and Australia, were also sent the survey link.This approach was supplemented by snowball sampling, with participants sharing the survey weblink with their colleagues and peers.Ethical clearance for data collection was granted by a dedicated panel at the University of Glasgow, College of Social Sciences.Data on participants' job title and length of rail industry experience was requested to evaluate their level of experience and, when combined with the high quality of responses provided to all other questions in the survey (e.g.all questions answered and detailed feedback provided in free text fields, using rail-specific terminologies), helped to confirm the validity of the input from all participants.All responses were voluntary and treated anonymously, with the survey being designed to prevent the identification of participants.The survey was pre-tested through distribution to three rail industry professionals.
To help focus survey responses, the question set was aligned with the three primary RQs, as shown in table 1.The ten NbS concepts included in the question set to address RQ1 were those which had been proposed by Blackwood et al (2022) as potential alternatives and/or complements to the CCA measures currently adopted by the rail industry to protect its infrastructure from or adapt it to accommodate climate change impacts.The ten concepts selected use existing NbS terminologies identified through a literature review conducted to establish NbS implemented in both rail and non-rail environments (refer to e.g.Transport for New South Wales 2017, Eisenberg and Polcher 2019), meaning that survey participants

All participants (n = 55)
Where participant confirms awareness of or involvement with the use of NbS on rail infrastructure 1. Are NbS being used as CCA measures in rail which are not included in the literature?
• Quantitative question to determine awareness of general CCA for rail infrastructure.
• Forced 1-4 Likert responses to determine levels of familiarity with ten NbS concepts.
• Funnelling of questions with quantitative and qualitative responses sought to obtain details on NbS usage.
2. What are the challenges and barriers to the operationalisation of NbS as CCA measures for the rail industry?
• Quantitative questions to establish current levels of CCRA undertaken and, where completed, their scope.may have awareness of the concepts and have been involved in their application outside of the railway context.Identifying railway examples of the application of the ten concepts, and/or confirming high levels of awareness of their application in rail would help to confirm the validity of the NbS alternatives and complements that have been suggested.
For questions on perceived NbS effectiveness 'I do not know' response options were provided.Whilst doing so may encourage satisficing (Krosnick and Presser 2010), because NbS are a new concept respondents may not have sufficient knowledge or information on which to form an opinion on the success of their performance.Further, due to the nature of rail infrastructure management, survey participants may only be involved in a short window of an asset's lifecycle (e.g. its design and/or construction) and therefore may not have visibility of NbS or CCA measures in use, particularly as some roles may be performed at considerable distance from the asset's location, meaning that a participant may genuinely not know how well an NbS performed.'I do not know' responses were treated as mid-point responses for the perception-based questions; otherwise, 1-4 scaled response options were provided in order to force an opinion, with low scores representing the lowest level of awareness or involvement, as appropriate.
Detailed descriptions of the ten NbS concepts were not provided in the questionnaire to avoid skewing results through participants claiming they were aware of a concept(s) they had not heard of previously.The provision of descriptions, however, may have enabled the distinction to be made between each concept, failure to do so does mean that people may have mixed up NbS concepts.Participants may therefore have stated awareness of the earliest featured NbS when later concepts may have been more appropriate.Even if responders realised this, they may not have known how to work back through the questionnaire to update their answers (even though this was possible) or wanted to take the time to do so.Participants may also have failed to recognise NbS concepts they are familiar with but refer to using a different terminology; by not seeing NbS nomenclature they are accustomed to included the questionnaire, some levels of awareness may be higher than indicated.
Data pre-processing was carried out using Microsoft Excel and statistical analysis performed using IBM SPSS Statistics (Version 28.0).A Shapiro-Wilk test determined that the data from Likert items were not normally distributed; a subsequent non-parametric Mann-Whitney U Test showed that there were statistical differences between the means of UK and Australian data.Thematic coding of qualitative data was conducted using NVIVO 12.

Results and discussion
Results are discussed below in alignment with the three primary RQs.Due to the purposive sampling employed, the 55 responses received were dominated by participants from the UK (60%) and Australia (33%); the majority of these were environment and sustainability professionals (53%), followed by those in project management (13%), and the Health & Safety and Engineering disciplines (each representing 7% of responses).The participants' organisations represent a range of rail industry functions and their associated supply chains, including infrastructure owners, operators and maintainers, along with rail infrastructure designers, constructors and support consultancies.Over half of the respondents had at least 10 years of rail industry experience, 27% of participants had 10-19 years' experience and a further 27% had over 20 years.Although there was a relatively small number of survey participants, reflective of the purposive sampling employed to target a very niche group of rail industry stakeholders, the quality of the responses received from experienced industry professionals contributes significant value to this research topic.Survey responses could only be submitted upon completion of all questions, i.e. there were no partially completed responses.Whilst quantitative data is presented in the following sections, the small sample size does however mean that the outputs remain largely qualitative.

RQ1 NbS being used as CCA measures in rail which are not included in the literature
The levels of participant awareness of ten NbS concepts are shown in figure 2. Using the means of the responses provided for each of the ten concepts to establish overall NbS awareness (i.e. the total level of awareness across all ten NbS), 11% of participants were aware of the concepts being used in rail, and 6% had been directly involved in the use of NbS as CCA measure on rail infrastructure, 63% of responders were aware of NbS concepts but not their application in rail, and the remaining 20% (1 in 5) had never heard of the concepts.
Survey responses demonstrated 61 instances of participants being aware of, and 35 cases of being directly involved in the use of NbS as CCA on rail infrastructure.Participants had most awareness of and involvement in the application of the natural drainage, green corridor and the use of vegetation to protect assets/infrastructure concepts.Green walls were the most recognised NbS with only 3 respondents having never heard of this concept; salt marshes and bioengineering/biotechnical solutions were the least familiar, with no known examples of use in the rail environment being provided for either.
Participants were asked to detail the location(s) of known NbS examples; some answers provided multiple locations per NbS concept, some provided none.Where locations were provided, some were specific (e.g.'Queen Street Tunnel, Scotland') whilst others were vague (e.g.'Australia-wide').As a further reflection of the purposive sampling method, most instances are either in the UK (66%) or Australia (28%).The locations are mapped in figures 3(a) and (b), with markers for answers that were quoted at a country or regional level being placed at the centre of the jurisdiction as per the geographic granularity provided in each response.Figure 4 provides an overview of the location of each NbS concept by country; one location described by a survey participant as 'abroad' has not been mapped.Of the 81 responses which included NbS locations, participants referenced 25 specific sites, i.e. at suburb level or named rail infrastructure location or asset, which enabled the identification of distinct NbS examples.This was not possible when a country or region-wide response was provided however, which means that there may be duplication in the recording of examples in figures 3(a) and (b).For example, one London NbS case may have been cited at the local city, England, Great Britain and UK levels, resulting in it potentially being counted up to four times.On the other hand, participants may have been aware of multiple examples at a regional or country level but did not reflect this in their response, leading to under-counting.The High Speed Two ('HS2') Route in the UK, for instance, was cited nine times across all survey replies; these have been recorded as nine separate responses rather than being rolled into one.Whilst this may be regarded as duplication, HS2 examples were referenced by three participants across five different NbS concepts, with one respondent providing a link to the HS2 'Green Corridor online mapping tool' which provides details of multiple examples of green corridor and associated NbS features along the length of the new high-speed line being constructed between the West Midlands and London, UK (HS2 Ltd 2022, n.d.).The new railway is currently under construction (HS2 Ltd 2021), meaning that many NbS are planned rather than operational, and therefore awareness of their presence may not yet be widespread, with the implication that, in later survey questions, responses cannot yet be given on their perceived performance.It should be noted that it may be easier to design and build NbS features into new infrastructure that is under construction, rather than retrofit measures into existing rail infrastructure.Given the land take that may be required to install NbS at a sufficient scale (The Royal Society 2014, Albert et al 2019, Sarabi et al 2019), the retrofitting of NbS (and/or other CCA measures) could be particularly difficult in urban areas, where space is at a premium (Sarabi et al 2019).The prominence of HS2 as the largest-scale rail infrastructure project currently underway in the UK introduces the potential for availability bias.The high number of NbS features included in the HS2 Green Corridor online mapping tool confirms the HS2 project to have significantly more documented NbS examples than have been found for other projects.As only three participants cited HS2 examples of NbS on rail infrastructure (representing 5.5% of survey respondents), their input is not regarded to have an unreasonable bearing or skew on the outputs of this research.Where participants responded that they had been directly involved in the use of NbS in rail, it could be assumed that they would be better placed to provide more detailed answers to the 'free text' questions.This was not necessarily the case.For example, 68% of respondents who had awareness of NbS in use provided a specific location, whereas only 61% of those who claimed to have been involved in the use of NbS concepts did so.Direct involvement, however, may infer greater confidence in the quality of responses.(Blackwood et al 2022) was heavy rail infrastructure, and did not include light rail or station buildings; the survey questionnaire did not feature these limitations however, and results included six references to light rail infrastructure; Birmingham New Street Station was cited four times, with answers not specifying whether they related to the building or rail infrastructure within its locale.
Whilst the limitations outlined above make it difficult to precisely confirm the number of NbS being implemented as CCA measures in the rail industry, the examples cited by survey participants indicate that, in answer to RQ1, there is a greater number, at a wider range of locations, than are currently included in the literature.It is likely that further examples of NbS are being used in the rail environment which are not recognised or labelled as such; these may include NbS concepts with different names to those applied in this study meaning that examples survey participants were familiar with were not recorded.Similarly, the limited inclusion of rail NbS examples in scientific or grey literature may be reflective of NbS still being a relatively new concept in rail and, where they are implemented, they are not documented as NbS (Blackwood et al 2022).Further, rail industry priority may be to initiate immediate operational responses to extreme weather events (Blackwood et al 2022) rather than spending time and money researching and writing about these; particularly if their responses do not deliberately include or explicitly reference CCA measures (Lindgren et al 2009).In addition to helping determine whether there were any (or any additional) live examples of use of the ten NbS concepts in the rail environment, the process of assessing their familiarity with each concept may have increased survey participants' awareness on the potential for NbS to be applied as CCA measures in rail, and/or this may have helped to demonstrate the potential transferability of the infrastructure protection and adaptation concepts from non-rail to rail scenarios which participants may not have previously considered.
Participants who were aware of and/or involved in the use of NbS in rail were asked to confirm the climate change hydrometeorological hazard(s) (HMH) in response to which each concept was being applied.Figure 5 confirms that high precipitation was the most addressed HMH (51 instances); this correlates with the literature review findings of Blackwood et al (2022).High temperature was the HMH with the second highest number of responses (33 instances), although it is recognised that it is the human-induced increase

RQ2 Challenges and barriers to the operationalisation of NbS as CCA measures for the rail industry
Questioning on the completion of CCRA established that 7% of participants never, and 15% rarely undertake these assessments during the planning, design, construction and/or operation of new or when upgrading existing railway infrastructure.With CCRA identified as a critical activity to incorporate CCA measures in rail infrastructure (Blackwood and Renaud 2022), this means that almost one in every four rail projects may not introduce CCA of any type to protect or adapt infrastructure.As well as raising concern over the long-term resilience of new infrastructure, this presents an immediate barrier to the uptake of NbS for this purpose.
A lack of awareness of the ten NbS concepts featured in the survey (figure 1 'I have never heard' responses) was verified via a direct question asking participants to select from a predetermined list their top three perceived barriers limiting the uptake of NbS.Responses are presented in table 2 where 'Lack of NbS awareness' is confirmed as the top barrier, with 20% of responses.Combining this with the 'Lack of NbS rail awareness' which received 6.7% of responses, it means that an overall lack of awareness on NbS concepts is the key barrier to the operationalisation of NbS as CCA measures, with over one quarter of rail industry survey participants citing this reason.Rail industry resistance to change was the second most selected barrier.The path dependency of stakeholders is a recognised barrier to the general deployment of NbS (Davies andLafortezza 2019, Frantzeskaki et al 2019), and the results of this survey help to confirm that the path dependence of railway engineers, with their resistance to changes to long-standing grey engineering traditions, is a particularly difficult sector-specific challenge for NbS to overcome (Blackwood and Renaud 2022).
Barriers to NbS uptake were also determined indirectly by asking participants with knowledge of or involvement in the implementation of NbS in rail (n = 61 and n = 35 examples, respectively) about any problems that they encountered during their use of each NbS concept.83 problems were cited from respondents located in three countries (n Other = 3, n Australia = 36, n UK = 44).Coded responses, collating answers from all ten NbS concepts, are presented in figure 6 which groups responses into 18 key themes.'Lack of awareness' did not feature in responses to this screened question directed only to those with knowledge or experience of applying NbS in rail but otherwise, with extra maintenance requirements and poor maintenance practices being the most commonly cited problems observed, and culture change also featuring heavily, these outputs again generally correlate with the top barrier findings shown in table 2 ('Rail resistance to change').
Recognising the contrasting climates, land use patterns and rail industry structures between and within the countries represented in this survey, responses confirm agreement from participants from all geographies on the top two barriers, and there was affirmation of barriers from both Australian and UK respondents over  13 of the 18 themes.The small number of responses received prevents detailed analysis of the results.However, general observations include that culture change and cost were regarded as bigger issues in the UK, whilst Australian participants highlighted the more technical practicalities of NbS design (i.e.capacity and structural considerations).

RQ3 Aids to the operationalisation of NbS as CCA measures for the rail industry
Over half of respondents 'Often' or 'Always' conduct CCRA when planning, designing, constructing and/or operating rail infrastructure or when upgrading existing assets, thus presenting the completion of CCRA as a strong mechanism to implement CCA in the first instance, and encourage that these include NbS.
Participants were asked to choose from a pre-determined list the top three measures they believe would enable the widespread uptake of NbS as CCA measures in rail; responses are presented in table 3 by country for Australia and the UK (table 3 data excludes the responses from four participants out with these locations).Even with the differing regulatory and rail industry structures implemented in each jurisdiction, table 3 shows that the use of Legislation, Policy and Standards and Client specification were the most selected options in both countries.These approaches align with the 'stick versus carrot' practice often observed in cultural change management, where prescribed, compliance-based requirements are set to mandate a change in behaviour in organisations with low sustainability maturity levels (Baumgartner and Ebner 2010).
A further means of aiding NbS implementation would be to promote the usage benefits reported by rail industry users, as gleaned from those with knowledge of and/or involvement in the use of NbS as CCA measures on rail.Coded feedback collated across all ten NbS concepts is shown in figure 7. 108 benefits were cited, from participants located in either Australia (n = 50) or the UK (n = 58).Biodiversity gains were the most reported benefit, followed by Aesthetics and Improved drainage.Coding of the survey responses identified 24 key themes versus 18 areas (figure 6) which, combined with the larger number of benefits than problems quoted (108 versus 83), can be seen as a reflection of the recognition of the multiple wide-ranging benefits of NbS.There is alignment in Australian and UK responses for the top 9 benefits, demonstrating the international recognition of these benefits.
Figure 7 confirms the multiple sustainability benefits that NbS can deliver, and, as suggested by Blackwood et al (2022), demonstrates that the use of NbS could therefore help the rail industry contribute to the United Nations 2030 Sustainable Development Goals (United Nations 2021), particularly 'Life on Land' , by using NbS to conserve, restore and sustainably use land and its ecosystem services to support the 'Industries, Innovation and Infrastructure' goal of providing reliable, sustainable and resilient infrastructure (United Nations Global Compact 2019).These additional benefits could also support rail's achievement of ambitious sustainability objectives including 'no net loss' and 'net positive' biodiversity targets (HM Government 2019).Promotion of these benefits could therefore significantly aid the operationalisation of NbS as CCA in rail.

Implications for the rail industry
The implications of the outputs of this research to the rail industry are discussed below, and are considered in relation to the three RQs.

RQ1 confirmed usage of NbS for CCA by the rail industry which is not included in the literature
Results from this survey are encouraging in that they confirm many examples of NbS being used to protect and adapt rail infrastructure in response to climate change, and that the rail industry is using the concept despite the extremely limited uptake suggested by the (lack of) literature on this topic.Because of the paucity of scientific or grey literature on the subject, however, there is limited promotion of the use of NbS as CCA for rail infrastructure, which may hinder its further uptake.This is confirmed by survey respondents citing a lack of awareness of the concept as being the biggest barrier to its uptake.Education of the rail industry on NbS is therefore a top priority.The further examination and documentation of the NbS examples cited in this research (where identifiable), and sharing of this information within the industry could assist with this.

RQ2 rail industry barriers to the operationalisation of NbS as CCA measures
The understanding of constraints is crucial to identifying ways of successfully delivering CCA measures and identifying adaptation opportunities (Nalau et al 2018).The challenges and barriers to the operationalisation of NbS as CCA measures for rail confirmed by RQ2 are valuable in this regard and should be addressed in the mechanisms adopted to roll out NbS in the industry; for example, rail standards and guidance for rail must address maintenance practices and requirements.Recognising the importance placed on the need for NbS education in the industry, training and awareness content should cover each of the constraints included in table 2 and figure 6; whilst sharing the many cited benefits of NbS (table 3 and figure 7), doing so will help to counter rail resistance to change, identified as a key barrier to uptake by 13.9% of survey participants.
The predetermined list from which survey participants could select their top perceived barriers to NbS uptake had been aligned with the seven barrier themes identified through literature review (Blackwood and Renaud 2022).The spread of questionnaire responses received across all twelve listed barriers (as shown in table 2) validates the literature review conclusions, whilst quantitative analysis of the survey responses provides a means of measuring the perceived significance of had previously been only qualitative findings; for example, 'Lack of cost benefit analysis' is regarded by more rail professionals to be a barrier than 'Climate change uncertainty'; this data could therefore be used by the rail industry to prioritise NbS enabling measures (as per table 3) to focus on the biggest apparent barriers first.Questionnaire responses on the problems encountered during NbS implementation (figure 6) have provided more practical and logistical perspectives on barriers to the use of NbS in the rail environment than were possible to identify through literature review, due to lack of literature on NbS being used in rail.Examples of the additional barriers this research has identified include difficulties in sourcing suitable plants, and security concerns due to the introduction of vegetation potentially contravening crime prevention through environmental design strategies, which are important measures for reducing crime on public transport, particularly in and around railway stations (Cozens and van der Linde 2015).It will therefore be important that enabling measures developed to support NbS roll out in rail address these barriers highlighted by the industry.
Further, regarding funding, the cost and lack of cost benefit analysis for NbS were cited as top barriers to their implementation by 7.9% and 10.9% of survey respondents, respectively, meaning that the financing of NbS is perceived as a challenge to their uptake by almost one fifth of participants.Expenditure would be required for a rail organisation to implement CCA of any type (grey or green).The greatest problem for the industry is likely to be at the point of planning CCA interventions due to uncertainty over how much NbS will cost over their lifecycle, and how this compares to traditional measures.NbS are disadvantaged by CCA decision-making being dependent on economic appraisal models customised to traditional, grey preferences (Chausson et al 2020), and difficulties in forecasting or measuring the effectiveness of NbS mean that there can be high uncertainty over their cost-effectiveness relative to alternative options (Rizvi et al 2015).In addition, Seddon et al (2020) note how 'inflexible and sectionalized forms of governance' mean that grey, engineered interventions remain default CCA options, thereby representing further hindrances to NbS uptake.This is likely to be the case for society in general, however, observation of this problem is expected to be amplified in the complex rail industry which involves multiple interconnected networks and stakeholders, whilst also being steeped in grey engineering traditions (Blackwood and Renaud 2022).This augments the recommendation for the more gradual phasing-in of NbS to rail organisations through green-grey hybrid solutions (Blackwood and Renaud 2022), which is supported by the growing consensus in the general roll out of NbS i.e. not specific to railways, that when comparing the costs and benefits of NbS against engineered approaches, hybrid solutions may present the best option in many contexts (Seddon et al 2020).The development of multi-criteria assessments which enable the costing and comparison of the multiple social, economic, and environmental co-benefits that can be derived from NbS is imperative to their universal roll out (Chausson et al 2020, Frantzeskaki et al 2019, Kabisch et al 2016, Ruangpan et al 2020).This does, however, present an opportunity for the rail sector to work with and learn from other industries to advance the tools and guidance to support the wholesale dissemination of NbS.In particular, railways could work with other transportation and linear corridor industries, such as highways and telecommunications, to develop shared solutions whereby collaborative approaches may also generate greater cumulative CCA benefits (Blackwood and Renaud 2022).

RQ3 rail industry aids to the operationalisation of NbS as CCA measures
Rail industry respondents in Australia and the UK agree that legislation, policy, standards and client specification will be the best mechanisms to support the wider implementation of NbS (table 3).To develop these instruments, set requirements and provide guidance to the industry, initial education and awareness are likely to be required by the legislation, policy, standard and specification setting bodies in both countries; this further emphasises the importance of and need for education.These parties will also need access to sufficient examples and robust evidence of NbS concepts in use to inform their development of the material required to set rail industry direction and guidance.With the setting of legislation and standards comes the matter of compliance and adherence.Seddon et al (2021) note that companies have historically failed to comply with voluntary environmental agreements, and therefore advocate rigorous assessment and validation through independent regulatory frameworks, supported by government policy.For the rail industry this would require clarification as to enforcement would sit with rail or environmental and planning regulators.
To provide the clear principles and evidence-based frameworks deemed necessary to aid NbS practitioners (Cohen-Shacham et al 2019), standards for rail NbS will need to be based on tried and tested examples, particularly from a rail safety perspective (Blackwood and Renaud 2022).The development of evidence-based criteria will not only support the design and implementation of NbS but also enable NbS commitments, relating to both climate change and biodiversity, to be monitored and improved with time (Seddon et al 2021).The construction of the new HS2 infrastructure in the UK, including the multiple NbS examples that are part of its 'Green Corridor online mapping tool' (HS2 Ltd 2022), presents an excellent opportunity to trial NbS concepts from which the rail industry could learn and use to develop policy, standards and specifications.Findings from HS2 would only be relevant to the UK and its climate analogues (Sanderson et al 2016), however any successful approaches and tools developed and/or lessons learned from site-specific case studies could be shared for application in the rail industry globally, for instance through dissemination by the International Union of Railways (UIC 2022).
'Landowner partnerships' was the measure least regarded as being an enabler to widespread NbS uptake, attracting only 1.9% and 3% of responses in Australia and the UK respectively (table 3).Such alliances, however, are likely to be instrumental in the successful implementation of NbS for rail for several reasons.Firstly, because the co-development of NbS with stakeholders, including landowners, is a key stage in the operationalisation of NbS (Kumar et al 2020).This step is presented as an essential activity in the 'validation of NbS options for rail' phase of the framework to incorporate NbS as CCA measures for rail infrastructure proposed by Blackwood and Renaud (2022).The failure of the rail industry to co-develop with adjacent landowners could therefore equate to failure of the validation of NbS options for rail, leaving the concept unable to get off the ground in the industry.Additionally, co-development through collaborative research and coproduction between scientists, practitioners, and the community is promoted as a means of advancing the planning and knowledge agenda for NbS (Frantzeskaki et al 2019).Involving stakeholders in joint 'Open Air Laboratories' (OALs) could also help to build a robust evidence base by demonstrating the effectiveness and sustainability outcomes of applying NbS compared to other CCA measures.For the rail industry, stakeholders should include lineside neighbours.These collaborative OALs could help to provide evidence to respond to questions or challenges raised regarding NbS performance uncertainty, lessen reluctance and cynicism in selecting natural solutions over engineered alternatives (Kabisch et al 2017) whilst potentially overcoming railway engineers' path dependency on traditional grey solutions.Furthermore, the limited amount of land directly available to rail infrastructure owners, i.e. generally long but very narrow corridors, presents a significant constraint to NbS uptake at scale (Travers et al 2021); this means that, without partnerships between rail and non-rail land owning neighbours, there may not be sufficient space to successfully implement NbS.The NbS concepts that survey participants had most awareness of and involvement with (natural drainage, green corridors and the use of vegetation to protect assets/infrastructure (figure 2)) are generally localised solutions, which would be more practicable to implement within the confines of the land owned by railways.Survey responses may therefore reflect this spatial constraint, with the lower levels of awareness and application of concepts such as reefs, mangroves and dune/beach restoration being due to these solutions requiring land take out-with the area of control of rail infrastructure owners.Finally, the formation of landowner partnerships would be critical to the realisation of the multiple potential community and social sustainability opportunities that joined-up NbS interventions may deliver, as detailed in figure 7.
The above points confirm that investigation into the formation of landowner partnerships represents a critical activity for the rail industry to undertake before embarking on the design, development and/or deployment of NbS.Use of the other enabling mechanisms listed in table 3 (e.g.education on the benefits of co-development and the provision of funding or incentives for this) could therefore be required to instigate collaborative NbS partnerships between rail infrastructure owners and their lineside neighbours to ensure that this happens.
It has been observed that very few studies on the adaptation performance of NbS have considered broader social, climate change mitigation, or, in particular, biodiversity outcomes, which have received only very basic coverage (Chausson et al 2020).The large number of global annual rail passengers (2.78 million passenger-km), lineside neighbours (along track lengths of 801 357 km), and rail industry employees (6 million) (International Union of Railways 2022) put railways in a prime position to access and monitor social sustainability data, including, for example, the social benefits quoted in survey responses as presented in figure 7: aesthetics ('pleasing and relaxing environment' , 'provision of shade, colour and aromas'); noise and vibration; air quality; social benefits; biodiversity ('access to nature').This provides the rail industry with an excellent opportunity to lead the way in collating data and learnings on NbS performance, and stakeholders' perception of their performance, which may be shared universally, internal and external to the sector.The quantification of the additional benefits gained through NbS implementation, as identified by participants of this study and shown in figure 7, would aid the promotion and subsequent operationalisation of NbS.Rail companies already undertake extensive vegetation surveys (for example, RSSB 2018, TEC Associates 2021, Network Rail 2022) representing a vehicle for the ongoing performance monitoring of plants being employed as NbS.However, ecologists would have to be deployed, and/or in-house staff trained up with basic ecological identification skills, in order to assess the wider biodiversity status of NbS locations, e.g. to validate increases in wildlife and habitats.Recognising that biodiversity gain is only one of the multiple benefits recognised by survey participants (figure 7), it is important that the multiple values of NbS, specifically the values of nature, are respected (Seddon 2022).There is otherwise a danger that token NbS are implemented as a means of greenwashing (Anderson et al 2019), with concerns already being raised that organisations are promoting their use of NbS whilst failing to take action to reduce fossil fuel consumption (Edwards 2020), and high greenhouse gas emitting industries, including airports, are using NbS to offset their emissions (Seddon et al 2021).Whilst rail is the least emissions-intensive mode of passenger transport (International Energy Agency 2023), as well as taking steps to introduce sustainable CCA measures through use of NbS, the industry must continue in tandem to pursue means of reducing its emissions, e.g. through the electrification of diesel operations, to support global climate change mitigation efforts.

Conclusion
There is a pressing need for railways to protect their infrastructure from and adapt it to accommodate the impacts of climate change.NbS have been recognised as potential CCA options for rail which may also deliver a range of additional ecosystem service benefits.To date, however, the widespread application of NbS by the rail industry has not been evident.Through engagement with international rail professionals, whilst limited by a relatively small sample size due the niche group of stakeholders that was targeted, this research has confirmed multiple examples of NbS in use in rail which are not included in the literature.The most commonly found NbS concept that is in use was natural drainage, which was predominately being applied in response to high precipitation events.
Many factors would have to be considered to support the widespread implementation of NbS in rail.This study has established that a lack of awareness of NbS is the largest perceived barrier to NbS uptake; education on and promotion of NbS in the industry will therefore be key to its successful deployment.Meanwhile, survey participants saw policy, standards, and client specification as the best vehicles to enable greater NbS uptake.To inform the development of these instruments, rail industry NbS case studies are recommended as a means of providing strong evidence and examples.OALs could be used to generate the required technical data whilst also offering live demonstration sites to educate stakeholders internal and external to the rail industry.
Encouragingly, this research has proven that rail professionals recognise the wide range of benefits that NbS can deliver in addition to their CCA function.This added value that natural solutions can provide, combined with evidence-based data and information that NbS can contribute effectively to CCA, should be used as an argument for the roll out of NbS as sustainable solutions for the rail industry, especially when making comparisons with traditional, engineered alternatives.

Figure 2 .
Figure 2. Survey participant awareness levels of ten NbS concepts.

Figure 3 .
Figure 3. (a) Distribution of NbS in rail examples in Europe.(b) Distribution of NbS in rail examples in Australia.

Figure 4 .
Figure 4. Locations of NbS concepts by type.

Figure 5 .
Figure 5. Natural hazards NbS are being used to address.

Figure 6 .
Figure 6.Problems encountered in NbS implementation on rail.

Table 1 .
Alignment of survey methods with research questions.

Table 2 .
Top barriers limiting the use of nature-based solutions as climate change adaptation options for rail.
in global surface temperatures that is affecting weather and climate extremes worldwide (IPPC 2021) i.e., high temperatures are exacerbating the other HMHs (except lightning) and therefore rail infrastructure responses to all other HMHs can infer an indirect CCA response to high temperatures also.Survey responses detailing the HMH(s) that each rail NbS example was being used to address generally align with the potential NbS applications suggested by Blackwood et al, strengthening the recommendations made for NbS alternatives and complements (2022).

Table 3 .
Top measures to enable widespread uptake of NbS.