Which factor primarily affects the grip of the train on the rail?

The grip of a train on the rail is primarily influenced by the wheel-rail interface. This factor encompasses the interactions between the wheels of the train and the rail track, including the surface conditions, contact geometry, and materials involved. The quality of the wheel-rail interface is critical because it determines the level of friction available for traction.

If the wheel surfaces are clean, well maintained, and the rail is in good condition, the frictional forces generated during acceleration, braking, and cornering will be optimal, significantly affecting the train's ability to maintain speed and safely navigate turns. Additionally, factors such as contamination from water, oil, or debris can adversely affect this interface, leading to reduced grip and potential slipping or skidding.

While train weight can influence the normal force at the wheel-rail contact point, making a heavier train exert more force downwards, it is the characteristics of the wheel-rail interaction that directly impact grip and, therefore, traction performance. Motor efficiency and control unit reliability, while important for overall train performance, do not directly influence the grip between wheels and rails.