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Third rail
A third rail, also known as a live rail, electric rail or conductor 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.
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).[clarification needed]
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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Third rail
A third rail, also known as a live rail, electric rail or conductor 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.
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).[clarification needed]
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.