Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems. 420 V DC, and a top-contact fourth rail is located centrally between the running rails at −210 V DC, which combine to provide a traction voltage of 630 V DC. Since the tyres do not conduct the return current, the two guide bars provided outside the running 'roll ways' become, in a sense, a third and fourth rail which each provide 750 V DC, so at least electrically it is a four-rail system. Most electrification systems use overhead wires, but third rail is an option up to 1,500 V. Third rail systems almost exclusively use DC distribution. Energy efficiency and infrastructure costs determine which of these is used on a network, although this is often fixed due to pre-existing electrification systems. The majority of modern electrification systems take AC energy from a power grid that is delivered to a locomotive, and within the locomotive, transformed and rectified to a lower DC voltage in preparation for use by traction motors.

Global Energy Network Institute. In 1970 the Ural Electromechanical Institute of Railway Engineers carried out calculations for railway electrification at 12 kV DC, showing that the equivalent loss levels for a 25 kV AC system could be achieved with DC voltage between 11 and 16 kV. Figure 3: Docklands Light Railway train with 3rd rail bottom contact electrification system. The additional rail carries the electrical return that, on third-rail and overhead networks, is provided by the running rails. The use of AC is usually not feasible due to the dimensions of a third rail being physically very large compared with the skin depth that AC penetrates to 0.3 millimetres or 0.012 inches in a steel rail. In 1919, what is electric cable Hubbell unsuccessfully tried to prevent other manufacturers from making receptacles and plugs to the dimensions used by Hubbell. This device, which can be a circuit breaker or the first outlet on a circuit, is designed to detect hazardous electrical arcing in the branch circuit wiring as well as in cords and plugs. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically bonded together.

Some of these, particularly Victorian mains that predated London's underground railways, were not constructed to carry currents and had no adequate electrical bonding between pipe segments. In some metropolitan areas, cables are enclosed by metal pipe and insulated with dielectric fluid (usually an oil) that is either static or circulated via pumps. Power-only rails can be mounted on strongly insulating ceramic chairs to minimise current leak, but this is not possible for running rails, which have to be seated on stronger metal chairs to carry the weight of trains. On "French system" HSLs, the overhead line and a "sleeper" feeder line each carry 25 kV in relation to the rails, but in opposite phase so they are at 50 kV from each other; autotransformers equalize the tension at regular intervals. Most stepdown transformers are designed for single-phase operation; if a three-phase secondary circuit is required, three physical transformers are usually required, and may be mounted the same pole as shown in the photo above. 1⁄2 in) standard gauge track between the roll ways operate in the same manner. The trains move on rubber tyres which roll on a pair of narrow roll ways made of steel and, in some places, of concrete.

This is mostly an issue for long-distance trips, but many lines come to be dominated by through traffic from long-haul freight trains (usually running coal, ore, or containers to or from ports). Regenerative braking returns power to the electrification system so that it may be used elsewhere, by other trains on the same system or returned to the general power grid. The higher the voltage, the lower the current for the same power (because power is current multiplied by voltage), and power loss is proportional to the current squared. DC rolling stock was equipped with ignitron-based converters to lower the supply voltage to 3 kV. An early advantage of AC is that the power-wasting resistors used in DC locomotives for speed control were not needed in an AC locomotive: multiple taps on the transformer can supply a range of voltages. While the work toward packing multiple telegraph signals onto one wire led to telephony, later advances would pack multiple voice signals onto one wire by increasing the bandwidth by modulating frequencies much higher than human hearing.