A magnetic track brake is commonly found on streetcars and light rail vehicles. It's basically a brakeshoe that works on the tracks. It has en electric magnet which "sucks" it to the tracks when activated. The magnetic track brake only has an on or off (all or nothing)position, and is used only in emergencies (slippery track conditions, or other conditions where the car needs to be stopped in a short distance. The magnet is usually powered by the car's low voltage system, i.e. also works when no high voltage from the overhead is present.
In Lisbon, tram drivers use this brake pretty often, since hills are steep, and the the only operating postitions used are either full power(or run 8 on a diesel) or full brake.
Boris