Factoring the wheel-rail contact into the brake control system
Conventional compressed air brake control systems regulate bogie brake cylinder pressure in line with brake demand and vehicle load information. The wheel slide protection system keeps wheel slip within the specified limits. However, braking/stopping distances are also influenced by (pressure control circuit and coefficient of friction) tolerances and by disturbance variables (environmental factors such as weather effects and track contamination) that are not input into the brake control system’s control circuit. In principle, the same problems also apply to electrodynamic braking.
A large-scale simulation study carried out in cooperation with Berlin’s Institut für Bahntechnik as part of the EU’s Shift2Rail program found that improved braking distance reproducibility has the potential to optimize rail infrastructure utilization, in some cases to a significant degree  (RBD). The theoretical headway reductions on a dry track were found to be in the following ranges: metros: 9% – 19%, S-Bahn trains: 9% – 16%, regional multiple units: 1.5% – 4%, and high-speed trains: up to 20%. The ranges for wet tracks were as follows: metros: 10% – 13%, S-Bahn trains: 10% – 12%, regional multiple units: 4% – 7%, high-speed trains: up to 20%. The variations in the theoretical headways are due to the different operating and line parameters and the difference between fixed blocks (Level 2) and moving blocks (Level 3) in the ETCS train control system.
Rather than allowing vehicles to brake harder, the aim of the integrated new deceleration control, wheel slide protection (WSP) and adhesion management systems is to increase the Emergency Brake Confidence Level by reducing braking distance variation. This makes it possible to reduce the headway between trains without compromising on safety.
Adhesion management plays an especially important role, since no other technology can deliver a sufficient improvement in (momentarily) extremely poor wheel-rail adhesion, especially with disc brakes. However, the development of adhesion management solutions for braking systems is complicated by the need to prepare entire track sections for the appropriate field tests. This makes the tests time-consuming, hazardous or even impossible
Consequently, test rigs such as Knorr-Bremse’s wheel-rail adhesion test rig ATLAS (Advanced Test Laboratory for Adhesion-based Systems) (Figure 1) play an extremely important role in the development and validation of first-time-right prototypes. ATLAS enables testing with original equipment at speeds of up to 350 km/h, under a wide range of reliably reproducible environmental conditions. The test-rig testing follows a V-shaped development model. Basic testing is followed by feasibility studies that are still decoupled from specific computer hardware. Finally, tests are carried out with product prototypes. This ensures that there is already a high degree of confidence when the first field test begins.