When it is time to stop a roller coaster the brakes have to be applied. Traditionally some sort of friction is applied to the wheel and axle system that turns the kinetic energy into thermal energy, slowing the train down. In most cases of roller coasters a combinations of electricity and magnets turns on and off the friction system that slows down and stops roller coaster trains.

Electromagnetism is when electricity is sent over an iron core that then becomes magnetized. Once an object is magnetized it attracts other metal objects. This is because when you send electricity through an iron object you change the way the electrons are behaving in the iron. The electrons will behave all the same way creating a magnetic field.

With this electricity can then be applied to an iron core turning on and off the magnetic field controlling when brakes can be applied and not to a roller coaster train.