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 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 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, at top contact or insulated brackets, at bottom contact, typically at intervals of around.The trains have metal contact blocks called collector 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.
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 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.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 rail and thereby also the loading gauge.
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 present a practical limitation on speed due to the mechanical impact of the shoe, and is considered the upper limit of practical third-rail operation. The world speed record for a third-rail train is 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, 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.
Weather effects
Third-rail systems using top contact are prone to accumulations of snow, or ice formed from refrozen snow, and this can interrupt operations. Some systems operate dedicated de-icing trains to deposit an oily fluid or antifreeze on the conductor rail to prevent the frozen build-up. The third rail can also be heated to alleviate the problem of ice.Unlike overhead line equipment, third-rail systems are not susceptible to strong winds or freezing rain, which can bring down overhead wires and hence disable all trains. Thunderstorms can also disable the power with lightning strikes on systems with overhead wires, disabling trains if there is a power surge or a break in the wires.
Gaps
Depending on train and track geometry, gaps in the conductor rail could allow a train to stop in a position where all of its power pickup shoes are in gaps, so that no traction power is available. The train is then said to be "gapped". Another train must then be brought up behind the stranded train to push it on to the conductor rail, or a jumper cable may be used to supply enough power to the train to get one of its contact shoes back on the live rail. Avoiding this problem requires a minimum length of trains that can be run on a line. Locomotives have either had the backup of an on-board diesel engine system, or have been connected to shoes on the rolling stock.Running rails for power supply
The first idea for feeding electricity to a train from an external source was by using both rails on which a train runs, whereby each rail is a conductor for each polarity, and is insulated by the sleepers. This method is used by most scale model trains; however, it does not work as well for large trains as the sleepers are not good insulators. Furthermore, the electric connection requires insulated wheels or insulated axles, but most insulation materials have poor mechanical properties compared with metals used for this purpose, leading to a less stable train vehicle. Nevertheless, it was sometimes used at the beginning of the development of electric trains. The oldest electric railway in the world, Volk's Railway in Brighton, England, was originally electrified at 50 volts DC using this system. Other railway systems that used it were the Gross-Lichterfelde Tramway and the Ungerer Tramway.Shoe contact
The third rail is usually located outside the two running rails, but on some systems it is mounted between them. The electricity is transmitted to the train by means of a sliding shoe, which is held in contact with the rail. On many systems, an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side or bottom of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides along the top surface, it is referred to as "top running". When the shoe slides along the bottom surface, it is less affected by the build-up of snow, ice, or leaves, and reduces the chances of a person being electrocuted by coming in contact with the rail. Examples of systems using under-running third rail include Metro-North in the New York metropolitan area; the SEPTA Market–Frankford Line in Philadelphia; and London's Docklands Light Railway.Contact shoe gallery
Electrical considerations and alternative technologies
Electric traction trains are considerably more cost-effective than diesel or steam units, where separate power units must be carried on each train. This advantage is especially marked in urban and rapid transit systems with a high traffic density.Because of mechanical limitations on the contact to the third rail, trains that use this method of power supply achieve lower speeds than those using overhead electric wires and a pantograph. Nevertheless, they may be preferred inside cities as there is no need for very high speed and they cause less visual pollution.
The third rail is an alternative to overhead lines that transmit power to trains by means of pantographs attached to the trains. Whereas overhead-wire systems can operate at 25 kV or more, using alternating current, the smaller clearance around a live rail imposes a maximum of about 1200 V, with some systems using 1500 V, and direct current is used. Trains on some lines or networks use both power supply modes.
All third-rail systems throughout the world are energised with DC supplies. Some of the reasons for this are historical. Early traction engines were DC motors, and the then-available rectifying equipment was large, expensive and impractical to install onboard trains. Also, transmission of the relatively high currents required results in higher losses with AC than DC. Substations for a DC system will have to be about apart, though the actual spacing depends on the carrying capacity, maximum speed, and service frequency of the line.
One method for reducing current losses is to use a composite conductor rail of a hybrid aluminium/steel design. The aluminium is a better conductor of electricity, and a running face of stainless steel gives better wear.
There are several ways of attaching the stainless steel to the aluminium. The oldest is a co-extruded method, where the stainless steel is extruded with the aluminium. This method has suffered, in isolated cases, from de-lamination ; this is said to have been eliminated in the latest co-extruded rails. A second method is an aluminium core, upon which two stainless steel sections are fitted as a cap and linear welded along the centre line of the rail. Because aluminium has a higher coefficient of thermal expansion than steel, the aluminium and steel must be positively locked to provide a good current collection interface. A third method rivets aluminium bus strips to the web of the steel rail.