This evaluation refers to the general potential for an increased share of regenerative braking in freight trains. Therefore, many evaluation categories are not applicable as such.
Regenerative braking in freight vs. passenger operation
Compared to passenger stock the potential for regenerative braking in freight trains is very limited. The main reason is the lower ratio of powered to unpowered axles.
- When comparing to passenger MUs this statement is obvious.
- But even comparing to loco-hauled passenger trains, freight trains have a disadvantage since they are much longer and heavier and have a larger mass to be braked by unpowered axles.
Limits to braking power of freight locomotives
The braking power of freight locomotives is limited by
- Adhesion: The maximum force that can be transmitted from the wheel to the rail without slipping (determined by axle load and rail conditions, e.g. on a track with wet leaves adhesion is lower for obvious reasons). Adhesion is difficult to influence.
- Longitudinal forces: If too much braking force is supplied by the locomotive, the heavy freight cars exert enormous pressure on the braking locomotive which leads to dangerous lateral forces in curves. Therefore maximum permissible longitudinal forces are defined limiting the use of dynamic brakes.
Strategies to increase the share of regenerative braking
- The most effective way of increasing the share of braking energy recovery in freight service would be increasing the number of driven axles. Based on the present concept of freight operation, this is obviously not realisable. Equipping usual freight cars with a motor for regenerative braking would be unaffordable.
- The innovative concept of self-propelled freight units would of course change the situation. However, due to the price advantages of diesel engines over electric motors and the operation on unelectrified routes, this concept has better chances to be realised with diesel traction.
- The introduction of differentiated limit values for longitudinal train forces.
- Swiss SBB have introduced radio-controlled double-traction in heavy freight trains.