|This article does not cite any sources. (May 2009) (Learn how and when to remove this template message)|
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.
Electromagnetic track brakes
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.
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.
Because of the risk of damage to the trackwork at points, etc., track brakes are typically 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, some rail systems (such as the MBTA Green Line in Boston, Massachusetts) use them much more frequently, to compensate for rails made slippery by weather conditions, or as a roll-back prevention measure on hills. Track brakes 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.
Cable car track brakes
The cable cars of San Francisco are fitted with mechanical track brakes, controlled by a large braking lever next to the grip lever. Pulling back on this lever forces replaceable pine wood blocks against the rails; as a result, a cable car descending a steep hill emits an odor of smoldering wood. These track brakes are routinely used many times while traversing a cable car route.
The true 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 transit line until the obstruction can be extracted with a cutting torch. The slot blade is controlled by the large red lever near the grip, and is not used except for emergency stops.
- Brake run for information on skid brakes and magnetic brakes.