Q: On electric trains, what stops the shoe which makes contact with the electrified track or the pantograph drawing power from the overhead cable from overheating? Presumably, tremendous heat is generated by friction alone. What stops the shoe or pantograph from seizing to the power source when the train stops?
A: To minimise heating and seizing at the interface of a sliding electrical contact, whether it is an overhead wire/pantograph or a contact rail/shoegear type, the contact resistance must be as low as possible. This is achieved by having a large contact area, a high pressure between collector and conductor and a clean conductor surface.
The surface of both rail and overhead wires is cleaned regularly by each passage of the collector. On high-voltage (25 kilovolt) overhead wires, the current is low (100 amps) so there is little heating effect, even with only one pantograph on the train raised. On lower voltage overhead lines (1.5 or 3 kilovolts), the current is higher (1000 amps) so the contact area is increased by using more wires and collector strips, both of which are larger than those used for high-voltage wires. On rail/shoegear systems, the voltage is lower (750 or 660 volts), the current higher (3000 amps) and an even larger collector surface is used, with many collector shoes in parallel.
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On high-voltage lines there is little friction because the graphite collector strips lubricate and polish the copper wire. With lower voltage systems, using copper collector strips on copper wires and steel collector shoes on steel rails, there is more friction, but once the train is moving there is adequate cooling of the collector shoes by the air flow.