How do Rehostatic brakes work?

Question

TGVs have them.

Answer 1

A Rheostatic brake is a regenerating brake.

A regenerative brake is an apparatus, a device or system which allows a vehicle to recapture part of the kinetic energy that would otherwise be lost to heat when braking and make use of that power either by storing it for future use or feeding it back into a power system for other vehicles to use.

It is similar to an electromagnetic brake, which generates heat instead of electricity and is unable to completely stop a rotor.

Answer 2

The electric motor that powers the train is turned of and used as an alternator like in a car, it takes away the potential energy and generates electricity at the same time.

Answer 3

The other answers above describe regenerative brakes, rheostatic is where the electric motors are used as generators to slow the train sometimes this current powers resistors on the train which turn electricity into heat hence slowing the train, the efficiency drops with speed so they cannot be used for stopping but can reduce wear on brake blocks very appreciably. I think  BR class 50s had this when new.

This can and frequently is used on Diesel Electrics where steep gradients are encountered, straight electrics with overhead wires or third rail usually feed the power back to the grid to assist other trains climbing the gradient. this is then called regenerative breaking.