Track brake
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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.
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[edit] Electromagnetic track brakes
[edit] Requirement
The grip of steel wheels on steel rails tends to be less than is the case with rubber tyres, though sanding 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 is fitted with electromagnetic track braking for emergency use. In a different context, some high speed trains, such as the Adtranz X2000 on Swedish State Railways, are fitted with the equipment, again for emergency use only.
[edit] Operation
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. They 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. In this way the shoe and track are forced together very strongly and powerful braking results. Because of the risk of damage to the trackwork at points, etc, they may be recommended for use only in emergency situations, typically if the tram loses control and sanding is ineffective, or the disc brakes have failed. In practice, however, some rail systems (such as the MBTA Green Line) use them much more frequently, to compensate for rail made slippery by weather conditions, or as a roll-back prevention measure on hills. They are usually operated by a driver’s emergency stop button mounted separately to the normal traction/brake controller, or by the release of the “dead man’s handle”; systems which use them frequently may have a separate switch to actuate the track brake without activating other emergency stopping measures.
On some systems permanent-magnet track brakes are used as parking brakes.
[edit] Cable car track brakes
The cable cars of San Francisco are fitted with mechanical track brakes, controlled by a large lever next to the grip lever. Pulling back on this lever forces pine blocks against the rails; as a result, a cable car descending a steep hill emits an odor of smoldering wood.
The emergency brake on a cable car is known as a "slot blade," a steel wedge that can be forced into the slot-rail between the running rails by a strong spring. If a runaway car is moving fast enough that the slot blade is necessary, the friction has been known to weld the blade to the slot rail, disabling the line until it can be extracted with a cutting torch. The slot blade is controlled by the large red lever near the grip.
[edit] See also
- Brake run for information on skid brakes and magnetic brakes.
