The operation of trams on a tramway track in Kosice, and their technical issues

Public passenger transport in the city of Košice is using the Vario LF2 + trams. However, there have been dangerous accidents of brake disc rupture during operation of the tram in operation and in the following article we analyzed the causes and suggested a solution to eliminate this dangerous phenomenon in the transport process.


Introduction
There in Košice the public passenger transport is besides others carried out by means of trams operated with Dopravný podnik mesta Košice (DPMK a.s.). The modernisation of old vehicles with new ones was financed with the EU funds through the Operational Programme "Transport" in the program period 2007-2013 and through the Operational Programme "Integrated Infrastructure" in the program period 2014-2020. There were 46 trams provided altogether. Technical issues began to show up in case of trams with serial numbers 841 and 832 [6].
A tram of a Vario LF2+ type is a six-axle two-unit tram intended for a one-way operation. Its prototype was manufactured in 2009. From the conceptual point of view it comes out from the Vario LF2 type, and the main difference lies in a new type of chassis thanks to which the high-floor part could have been lowered. Both entrances between chassis of the vehicle are barrier-free, and there is one step at the marginal entrances into the vehicle. Fig. 1. represents the tram of the Vario LF2+ type, and technical parameters of the vehicle are listed in Table 1.

Description of the Tram
The support structure of VarCB3 -LF2+ body is welded; it is a light steel construction with an anticorrosive finish with laminated heads of Pelikán design [15]. The outermost body-sides are made of polyester glass laminate mounted via a glue application on the body shell. The head sections of outermost units are reinforced. The cabinets of the tram are carried with three driving chassis Komfort+ which are conceptually designed as rotational ones with a two-stage spring loading and a bolster which handles a mechanical coupling of the cabinet with the chassis. The coupling of cabinets on the middle chassis is at the same time created with a tram joint and it carries the interior parts of both tram cabinets.
Each chassis is driven with two asynchronous engines. The current collection is provided with a half-pantograph Lekov EPDE 17-2600. In case of braking the energy is recuperated to auxiliary consumptions of the vehicle and to the trolley network with priority. On the roof of the vehicle there are 3 containers of drive placed in total; on the unit A there is one container, and on the unit B two containers provide the power supply of traction engines.
The entrances of outermost units from outer sides of chassis have one step which is 250 mm high; a sill is 350 mm high from the top of the rail. The other entrances have the entry level height of 350 mm from the top of the rail and they are without any steps. The interior steps there in passages of A and B units and there above chassis are created with one step overrunning a difference in height of 190 mm. A low-floor part creates 43 % of the overall floor area of the tram. All windows on the body feature a structural glazing; windows away from the vehicle's head and windows with monitoring panels are sliding ones. There are 4 doors serving for the entrance into the vehicle. The interior of the vehicle is equipped with anti-slip flooring and with seats STER MX with textile upholstery covers [6]. Technical parameters of the tram are represented in Table 1.  15 April 2019 and 29 April 2019; step by step it analysed all available materials: recordings from cameras in the vehicles, records from the tachographs, records from the vehicles' GPS, records from the diagnostic devices, records on failures, results of metallurgic tests of the brake disc, witness statements of respective employees of DPMK, a.s., and other expert data regarding the incidents in order to reach the reason for incidents in the rail vehicles of the VARIO LF2+ type with the Reg. No. 841 and No. 832 [2,14]. On 29 April 2019 a burst test was performed on the vehicle with the Reg. No. 832 in order to verify the function of protective elements preventing any part of the chassis to get to the interior of the vehicle. During the burst test the audio and light alarm of the drive with non-released brakes of the chassis was working.
The committee agreed on facts which could have led to the destruction of the brake disc ( Figure 2). The most probable initiatory defect was a leakage in traction engines' terminal boxes, caused with splashing water from vehicle's chassis wheels. This fact is also supported with the state of the railway tramway track in Figure 3. There was no alarm signalling the failure of brakes, however, a failure of the engine unit 3 was signalled. Thus, the tram driver did not pay attention to the failure and continued in the drive with two traction units on two chassis. Besides the monitors of the diagnostic system often display unreal failures which the drivers use to ignore in the course of time. A drive with a stalled chassis should definitely be prohibited, but the manufacturer of the electric part, Škoda Electric, enables it in order for a tram to get out of a crossing or even to the depot for a repair in a state of emergency. This failure was provably detected; there on multiple sections of the tram's drive the brakes were applied and they stopped working after some time. The tram's drive enables a passage even in case of a higher water level up to the depth of 60 mm on the tramway, but in fact there even occurred conditions with the water depth up to 100 mm. In Figure 4 we can see the placement of the brake disc on the tram axle and the placement of a protective cover above the brake disc.
Based on the inspection of the suggested protection of the tram floor there were also conducted mathematical-model re-calculations of resistance of the protective stainless plate below the floor per the software support in Abaqus program, which are represented in Figure 5 [1,4,5]. Subsequently on the basis of an expert analysis of the given materials the following solutions were suggested; they should ensure a safe and smooth operation of vehicles in DPMK. The following measures were suggested:  to carry out a replacement of terminal boxes for traction asynchronous engines on individual chassis of vehicles with the Reg. No. 832 -846 and to prevent water leakage into them [9],  a technological and geometric adjustment of the track was suggested in order to remove steady water on the tramway,  upon the approval of the SW update in the Ministry of Transport and Construction of the Slovak Republic and the subsequent installation into vehicles to unlock the SW of vehicles for the speed of 65 km.h -1 ; the handbook of vehicles was updated,  new technological procedures were implemented for each type of vehicles' maintenance, upon the consultation with the manufacturer to re-evaluate operations and km run for the sake of the maintenance of the brake disc M1,  to update all technological procedures for each type of maintenance of the rail vehicle, VARIO LF2+ type, upon the consultation with the manufacturer [7,8],  due to a potential destructive state of the brake disc there was a cover plate installed on the floor below the vehicle. This cover plate was mounted on all vehicles. A cover plate is a sheet with 3 mm in thickness made of austenitic corrosion proof steel X5CrNi18-10, Figure 6 [13],  a check of the ERB2 brake control unit's setting was conducted and a function of an increased voltage for a brake valve was added [10,11,12].  Figure 2. The brake disc damaged with cracks source.

Conclusion
On the basis of detected facts the cause of the failure could have been identified and removed. Such failures have not occurred anymore since this adjustment and the trams mentioned above have been driving without any failures so far.