Slows down trains, because the default speeds are silly.
Instead of a base speed of 259 km/h, the base speed is 86 km/h (1/3 of default). Acceleration is slowed, weight is increased, and braking force is reduced. Trains take longer to speed up and slow down, and don't go as fast overall.
Braking force research still reduces the braking distance. Advanced fuels still increase the acceleration and maximum speed, but below the vanilla values.