[jimktrains2]

Just like on a car, if your break pads are capable of completely stopping the wheel, but the wheel is not capable of creating enough braking force with the surface, then you slide. If you lubricate between the wheels and surface, which is essentially what leaves do on rails, no amount of at-wheel breaking power will stop you.

This is what antilock Brakes in cars prevent, they pulse the brakes to allow the tire to regain traction, preventing slippage and loss of control.

[tialaramex]

Note that a modern train also has the same system, only more so, a car would typically have 4 wheels, I think the Acela probably has like 96 or something.