Third rail

A third rail, also known as a conductor rail, electric rail, live rail, or power rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost fully segregated from the outside environment. Third-rail systems are usually supplied with direct current. Modern tram systems with street running avoid the electrical injury risk of the exposed electric rail by implementing a segmented ground-level power supply, where each segment is electrified only while covered by a vehicle which is using its power. The third-rail system of electrification is not related to the third rail used in dual-gauge railways. The system is generally associated with a low voltage (rarely above 750 V) and is far less used for main lines than overhead line which, with a higher voltage, permit more distance between the substations. Also, for safety reasons, third-rail systems are generally fully grade-separated. Third rail found its niche in metro systems, where a smaller tunnel is more important than having fewer substations; however, some main lines use third rail, like lines in Southern England, Merseyrail, Long Island Rail Road, Hudson and Harlem lines of Metro North Railroad, and Mitre, Sarmiento, and Urquiza lines in Greater Buenos Aires.

Description Third-rail systems are a means of providing electric traction power to trains using an additional rail (called a "conductor rail") for the purpose. On most systems, the conductor rail is placed on the sleeper ends outside the running rails, but in some systems a central conductor rail is used. The conductor rail is supported on ceramic insulators (known as "pots"), at top contact or insulated brackets, at bottom contact, typically at intervals of around 10 feet (3.0 m). The trains have metal contact blocks called collector shoes (also known as contact shoes or pickup shoes) which make contact with the conductor rail. The traction current is returned to the generating station through the running rails. In North America, the conductor rail is usually made of high-conductivity steel or steel bolted to aluminium to increase the conductivity. Elsewhere in the world, extruded aluminium conductors with stainless steel contact surface or cap, is the preferred technology due to its lower electrical resistance, longer life, and lighter weight. The running rails are electrically connected using wire bonds or other devices, to minimise resistance in the electric circuit. Contact shoes can be positioned below, above, or beside the third rail, depending on the type of third rail used: these third rails are referred to as bottom-contact, top-contact, or side-contact, respectively. The conductor rails have to be interrupted at level crossings, crossovers, and substation gaps. Tapered rails are provided at the ends of each section to allow a smooth engagement of the train's contact shoes. The position of contact between the train and the rail varies: some of the earliest systems used top contact, but later developments use side or bottom contact, which enabled the conductor rail to be covered, protecting track workers from accidental contact and protecting the conductor rail from frost, ice, snow and leaf-fall.

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Advantages and disadvantages

Structure gauge For the same vehicle size, third-rail electrification requires a smaller vertical structure gauge as compared to overhead line electrification. This consideration becomes especially important for urban underground railways, where a smaller structure gauge allows for smaller tunnel cross sections and corresponding savings on construction cost.

Safety

Because third-rail systems, which are located close to the ground, present electric shock hazards, high voltages (above 1500 V) are not considered safe. A very high current must therefore be used to transfer adequate power to the train, resulting in high resistive losses, and requiring relatively closely spaced feed points (electrical substations). The electrified rail is a hazard to anyone on the tracks. The risk can be mitigated by using platform screen doors, or by placing the conductor rail on the side of the track away from the platform, when allowed by the station layout, or by covering the conductor rail with a coverboard, a plank supported by brackets. However, coverboards often cannot be used because they reduce the structure gauge near the top of the rail and thereby also the loading gauge. Chicago "L" and MBTA Subway third rails are not covered. There is also a risk of pedestrians walking onto the tracks at level crossings and touching the third rail, unless grade separation is fully implemented. In the United States, a 1992 Supreme Court of Illinois decision affirmed a $1.5 million verdict against the Chicago Transit Authority for failing to stop an intoxicated person from walking onto the tracks at a level crossing at the Kedzie station in an apparent attempt to urinate. The end ramps of conductor rails (where they are interrupted, or change sides) present a practical limitation on speed due to the mechanical impact of the shoe, and 161 km/h (100 mph) is considered the upper limit of practical third-rail operation. The world speed record for a third-rail train is 175 km/h (109 mph) attained on 11 April 1988 by a British Class 442 EMU. In the event of a collision with a foreign object, the beveled end ramps of bottom-running systems can facilitate the hazard of having the third rail penetrate the interior of a passenger car. This is believed to have contributed to the death of five passengers in the Valhalla train crash of 2015. Modern systems, such as ground-level power supply (first used in the tramway of Bordeaux in 2003), avoid the safety problem by segmenting the powered rail, with each segment being powered only when fully covered by the vehicle which utilizes its power.

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