third rail

Article

July 5, 2022

Third rail is a method of electrifying a railway line, metro line or tram line, as a counterpart to the overhead wires. The third rail is almost always supplied with DC voltage. The most common voltage is 750 volts.

Third rail along the side of the track

This type is mainly used on railway lines in southern England, by Merseyrail and on metros. With the Paris and London metro and the four small profile lines of the Berlin metro, the power is taken at the top. Outwardly, the supply rail is virtually indistinguishable from the rails on which the wheels run. In addition to the safety risk, snow and ice on the rails are very disruptive to the current draw. In South East England special trains run during snowfall to keep the power rails clean. On newer systems, the power is taken off with a trailing shoe at the bottom for safety reasons. The Amsterdam, Brussels and Rotterdam metros, among others, are equipped in this way. See below, electrocution hazard.

Third and fourth rail

In London and Milan, a fourth rail is also used, which lies between the rails. Stray current corrosion can be avoided with such a separate return flow line. On pneumatic tire metros, such as the Lyon metro, an upright rail is fitted on both sides. These rails are not only used for the power supply, but also for the guidance of the vehicles.

Third rail between the rails

This type was used a lot on French city trams in the past. An improved version of this system is reused on the Bordeaux tram network under the name Alimentation par le sol (APS). At APS, the third central rail is divided into different segments that are only energized when a tram runs over it. In certain systems with vehicles on pneumatic tires, such as a TVR, VAL or people mover, the third rail not only serves as a power supply, but also to guide the vehicles.

Model trains

On the Märklin and Trix Express model trains, there is a third rail between the rails. At Märklin, the third rail is poorly visible because it consists of small dots (approximately since 1960). An elongated trailing shoe under the locomotive takes the power off.

Third rail above the track

The Madrid metro and the Barcelona metro, among others, are fed from above. However, there is no slack wire but a rigid rod, so a mixture of overhead wires and third rail. Because the rod is immobile, a less high tunnel is needed. At the Wuppertaler Schwebebahn, all rails are at the top.

Advantages and disadvantages of third rail compared to the overhead line

Advantages: A limited gauge that allows cheaper, lower tunnels to be constructed. That is the main reason why third rail is usually preferred for subways. Less susceptible to interference (no wire breaks or catenary broken by the pantograph) No posts and wires, therefore cheaper to install and less unsightly Disadvantages: More dangerous due to risk of electrocution Limited capacity, due to the proximity to the ground, the voltage must remain low Poor contact and poorer current draw in snow and ice. However, catenary can also suffer from ice build-up (icicles). The second disadvantage makes a third rail impossible for long-distance railways and high-speed lines, where the high speed requires a high supply voltage.

Electrocution Hazard

A railway or metro line is fenced and not accessible to the public. As a result, the risk of electrocution is not particularly great. Level crossings – which are rare on railways equipped with a third rail – present an additional hazard. The third rail is interrupted at the level crossing. A risk is the possibility of walking along the track at a level crossing. At platforms, the third rail is usually on the side facing away from the platform, but it still remains dangerous