Rail E-SSENTIALS

No more steering clear of “on-track” tests in rail traffic

In dynamic running testing, producers/distributors and operators must demonstrate that their trains can safely pass through defined curve radii above the allowable maximum speed and do not damage the line. However, there is often a lack of suitable rail sections for the tests. A comprehensive safety concept opens up new test routes.

Compared to other types of traffic, rail traffic achieves a relatively high safety level through numerous safety precautions and system-related safety mechanisms. To maintain this, new vehicles are tested to the specified standards during the type tests before they are approved and licensed for in service operation. The fact that each vehicle unit runs safely without exerting excessive loading on the track essentially depends on its dynamic running behaviour. The testing of running characteristics is an important part of the approval process, which must be taken into account during the development of a new rail vehicle. Simulations, for example, can be used to optimise the vehicle dynamics at an early stage, and the simulation data acquired can also – according to new provisions such as EN 14363:2019 – replace the usual measurement data, at least partly. This can reduce the duration of cost-intensive on-track tests, under certain circumstances.

The on-track tests and the evaluation of the dynamic running behaviour are performed by independent test bodies. During these tests it is necessary to demonstrate that the limit values of running safety and track loading defined in the European standard EN 14363 are met. This ensures that the train neither derails nor displaces the tracks or otherwise overloads them.

Evaluation of running safety

The weight and load distribution on the vehicle’s wheelsets are significant for the vertical wheel force (or wheel load, denoted with Q), which acts vertically between the rail and rail. The lateral guiding force (Y) is measured transversely to the running direction. Both variables are decisive for the evaluation of running safety. Under certain circumstances, if the sum of these guiding forces is too large, the track can be moved. If the lateral guiding force exceeds the vertical wheel force significantly, the vehicle could derail due the wheel flange riding up over the running surface of the running rail. The safety-related limit value of the guiding force therefore depends on the vertical wheel force. The basic rule is the greater the value of Q, the higher the limit value for Y also. At the same time, the quotient of guiding force and wheel force (Y/Q) must not exceed 0.8 for a curve radius of more than 250 m. Maximum 1.2 is allowed in exceptional cases.

Both forces also play a central role in the evaluation of the track loading. Excessive rail wear would drive up infrastructure maintenance costs. Therefore, limit values for the quasi-static guide force and the quasi-static wheel force apply. “Quasi-static” is the measured value that remains constant for a lengthy time in the dynamic running test. The maximum wheel force may also not be more than 90 kilonewtons (kN) above their quasi-static value.

Taking into account vehicle dynamics

An acceleration transverse to the running direction acts on a rail vehicle negotiating a track curve (centrifugal acceleration). It depends on the radius (or radius of curvature) and the vehicle speed. It can be partly compensated for by a structural track cant. To do this, the rail on the outside of the curve is raised or rather the inner curve rail is lowered. However, the lateral acceleration cannot be completely eliminated, as different vehicles negotiate the curve at different speeds and may also not overturn in case of a standstill in the curve. The amount of cant therefore depends on the curve radius and the maximum running speed. The cant theoretically necessary to balance out the lateral acceleration can be calculated and is called the cant deficiency.

EN 14363 divides the on-track running tests into four test zones with different section lengths and curve radii. Different requirements apply to the speed and the cant deficiency of the vehicle to be tested.

The stability of the vehicle at maximum speed and real wear pattern in service are examined in the first zone. Stability problems tend to occur at hight speeds on a straight section, therefore, this zone mainly has straight sections, which the vehicle must negotiate with a speed ten percent above the maximum speed targeted for the approval. The vehicle’s behaviour in track curves is evaluated in the other test zones. Test zone 2 contains large curve radii (over 600m), which must be negotiated with up to ten percent above the vehicle’s maximum speed. Large centrifugal forces now act on the vehicle, which is why the dynamic fractions of the evaluation variables are mainly of consequence here. In test zones 3 and 4, the vehicle is tested at lower speeds in track curves with small (400-600 m) and very small (250-400 m) radius. The dynamic fractions in the evaluation characteristics tend to be low, the quasi-static fractions in the guiding and wheel forces, on the other hand, increase. In test zones 2 to 4, a cant deficit of up to ten percent above the approval value is targeted.

The required routes must be sought by the network operator and evaluated during the test planning. An ideal test route includes straight route sections and numerous curves with different radii, so that as many of the test zones described above as possible can be covered and to avoid having to change routes. The precise properties and minimum length of the test routes depend on the approval parameters of the vehicle. Under certain circumstances, a very small number of suitable routes can result from the sum of the requirements. In addition, operational, construction or formal reasons often reduce the choice. The time and cost of the on-track tests therefore increase significantly. Clever selection of the test method, comprehensive planning and a well thought-through safety concept can minimise this time and cost and open up new options.

Case example: On-track test in exceptional case

RailAdventure GmbH, a Munich company specialised in performing test runs in rail traffic, was engaged to perform on-track tests for a Norwegian high-speed train. This was to be approved for a maximum speed of 245 kilometres per hour (km/h) and therefore had to navigate curves with radii of less than 3,000 metres with approx. 270 km/h. Following intensive discussions with the network operator, the curves required by the standards were found in Germany, however, the contact line system on this route section was only approved for speeds up to maximum 230 km/h. High speeds influence the contact force of the current pantograph significantly and thus also the contact wire uplift. If this increases too much, the overhead contact line can be damaged. In addition, the Norwegian train exceeded the usual static load gauge in Germany and did not have any German train command/control systems. RailAdventure therefore had to demonstrate to the operator that the allowable maximum speed of the route could safely be exceeded for the test. Together with the experts of the project partner TÜV SÜD Rail GmbH and working in close cooperation with the network operator, a concept was developed which should enable the trips.

In the first step, simulation calculations were performed and evaluated of the interaction of the current pantograph with the overhead contact line at overspeed. After the signals for the on-track test were set to green, in the next step it was necessary to ensure that the contact wire uplift remains within the allowable limits during the test runs. The uplift is measured with the vehicle stationary in only a few points on the route, therefore they must represent the whole route. Track test vehicles ran along the route to find these critical places. The current pantograph with Norwegian profile (1800 mm instead of 1950 mm width) were then optimised together with the manufacturer – in tests on a route approved for these speeds – so that the forces that occur at 270 km/corresponded to those of normal running at 230 km/h. In this way, the overall behaviour was optimised so that, despite the increased speed, the contact wire uplift should remain within the allowable limits, even in the most critical point on the route. As a further safety measure, the vehicle runs were undertaken in single traction without only one current pantograph. This minimises the wave movement of the overhead contact line compared to a run with multiple raised current pantograph.

The running tests on the selected route began following this optimisation phase. After initial runs with the allowable maximum route speed of 230 km/h, the speed was gradually increased to the maximum speed – with an eye on the measurement results for the rail and overhead contact line contact points.

However, the contact force at lower speeds was now below the normative tolerance range. Therefore, further steps were then necessary so that the current pantograph complied with the approval-relevant limit values. The special aerodynamic optimisation was decisive, to be able to exactly set the contact force between the contact strip and the contact wire for the test runs.

All measures together enabled the required on-track tests to be completed without excessively loading or even damaging the current pantograph, the overhead contact line and the route. The Norwegian train was therefore able to be successfully approved, although originally no suitable route had been available. The example shows how, with adapted methods, comprehensive project planning and technical expertise, distributors/manufacturers and operators can acquire a larger choice of test routes for testing high-speed trains in Germany and Europe.

For more information, please contact our experts:

Dr.-Ing. Thomas Noack, TÜV SÜD Rail GmbH, Prüfstelle Schienenfahrzeuge

Dipl.-Ing. Jörg Schurig, RailAdventure GmbH, www.railadventure.de