Magnetic brakes are unique to rail borne vehicles. Powerful electromagnets, whose force can be controlled by varying the electrical current through them, clamp themselves to the rails by magnetism. They are independent of the wheels, which can still be used for normal braking.
The electrical power for these magnets is never derived from the normal traction supply in case this should fail at a critical moment. Older trams used to switch the motors so as to act as generators for the brakes, the braking force depending on speed and giving an element of automatic control. Modern trams have a reliable battery supply for the lights and other essentials; this is used to supply the magnetic brakes, which are nowadays very powerful and used only in emergency.
Magnetic brakes must not be used on points or crossings, except in the direst emergencies, because they might catch the point tongue or snag the crossing point and rip up the track or tear the brakes or bogies off the tram. In any case they would not be fully effective because the magnets are not as strongly attracted to the type of cast steel used for this special trackwork as they would be to ordinary rail.
Photograph of modern magnetic brake (98kb)
Drawing of Westinghouse Electromagnetic Track Brake of 1904 (33kb)
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