The third rail is usually located outside the two running rails, but occasionally 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 (called side running) or bottom (called bottom running) of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides on top, it is referred to as top running. When the shoe slides on the bottom it is not affected by the build-up of snow, ice, or leaves.
As with overhead wires, the return current usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tyres, as on parts of the Paris Métro, Mexico City metro and Santiago Metro, and on all of the Montreal Metro, live must be provided to feed the current. The return is effected through the rails of the conventional track between these guide bars (see rubber-tyred metro). Another design, with a third rail (current feed, outside the running rails) and fourth rail (current return, half way between the running rails), is used by a few steel-wheel systems, see fourth rail. The London Underground is the largest of these, (see railway electrification in Great Britain). The main reason for using the fourth rail to carry the return current is to avoid this current flowing through the original metal tunnel linings which were never intended to carry current and which would suffer corrosion should such currents flow in them.
On line M1 of the Milan Metro the third rail is used as the return electrical line (with potential near the ground) and the live electrical connection is made with a sliding block on the side of the car sliding on an electrical bar parallel to the track approximately 1 m (3.3 ft) above rail level. In this manner there are four rails. In the northern part of the line the more common overhead line system is used.
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 (AC), the smaller clearance around a live rail imposes a maximum of about 1500 V (Line 4, Guangzhou Metro, Line 5, Guangzhou Metro, Longgang Line, Shenzhen Metro), and direct current (DC) is used. Trains on some lines or networks use both power supply modes (cf. below, "Compromise systems").
One method for reducing current losses (and thus increase the spacing of feeder/sub stations, a major cost in third rail electrification) 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 (where the stainless steel separates from the aluminium); 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 aluminum bus strips to the web of the steel rail. The photo below-right depicts such a rail.
Read more about this topic: Third Rail
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