My wife and I saw a movie called "Unstoppable" this evening. The premise is how a run away train is stopped. It is based on a real event.
Modern locomotives have what appear to be a set of wings on the rear of the lococomotive with a large grille on top of the wings. These are used for "dynamic" braking. Basically the electric drive motors are run backwards as generators/brakes sending current to a huge array of resistors under the grille. This dissipates the energy as heat.
A key point in the movie is when a person suggests using this as the primary means of braking when a spare locomotive is attached to the end of the train to slow it down. He points out that the static friction of the rolling wheels is preferable to ordinary mechanical braking that would slide the wheels along the track when the wheels stopped turning. It was nice to see a movie get it right.