On: Why do railway tracks have crushed stones alongsid...

[Symbiote]

Many trains, including diesel trains, use the motors as generators for braking. Electric trains might return the power to the grid, diesel ones can dump it into a large bank of resistors.

https://en.wikipedia.org/wiki/Dynamic_braking

[SoftTalker → Symbiote]

To be clear, even diesel locomotives use electric motors to drive the wheels.

The diesel engines drive an on-board generator to make electricity.

This avoids the need for a transmission, which would have trouble delivering the torque needed to start the train from a standstill.