Track brake

Track brakes are a form of brakes unique to railborne vehicles. The braking force derives from the friction resulting from the application of wood or metal braking shoes directly to the tracks (as the name suggests). Early examples of track brakes used on the horse hauled mineral tramways that preceded the steam locomotive were described as sledge brakes, and are usually associated with lines that used gravity propulsion.

Early systems used manual force to apply the braking shoes; more recently system have used arrays of electromagnets to hold the shoes against the rail. In some applications, the shoes are applied by powerful springs, and held off by mechanical or electro-magnetic force.

The grip of steel wheels on steel rails tends to be less than is the case with rubber tyres, though sanding (dropping gritty material on the track) does compensate if wheel slippage occurs. Therefore when light-rail systems or trams share space with pedestrians, automobiles and other road traffic, or where the vehicles operate on steep track, safety demands that the tram be fitted with electromagnetic track braking for emergency use.

In a different context, some high-speed trains, such as the Adtranz X 2000 on Swedish State Railways, are fitted with the equipment, again for emergency use only.

The power of electromagnetic track brakes comes from electromagnetic attraction between the brake and the track. They are intended to provide retardation beyond the adhesion limit of the wheels alone, which ultimately is limited by the weight of the vehicle. Track brakes are fitted on the bogies between each pair of wheels and in line with the running rails. In operation they are first dropped into position on the rails, using air actuators, and then current is applied to strong electromagnet coils within the shoes. This pulls the brake shoes hard against the track with a force that can exceed the vehicle's weight, and strong braking forces result.

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Wikipedia