The other shoes on the same circuit must be isolated while this is done, unless the current is switched off from the whole section - perhaps disabling several other trains. The return circuit is via the running rails back to the substation. This allows for some over supply and provides for continuity if one substation fails. Someone muted the idea of erecting an electric pole both at the supply source and next to the T in question but this was a non-starter for safety and aesthetic reasons. This pole supports: - Three-phase open-wire primary electric distribution. Starting at the top and working down, the facilities in the supply space are: Static wire: a grounded wire at the very top of the pole intended to protect lower conductors from lightning. Top contact systems with protective covers over them, like the New York Subway (Figure 4), needed radially mounted shoes anyway to allow them to fit under the cover. More recently, mechanical or pneumatic systems have been devised to make it possible to lift shoes from inside the train remotely from the driving cab.

The power losses can be reduced by locating shoes along the train and connecting them together by a cable known as a busline. Most types of top contact shoes simply hang from a beam suspended between the axleboxes of the bogie. The shoe is suspended from an insulated beam hung between the axleboxes. Isolation used to involve inserting a wooden "paddle" between the shoe and the current rail and then tying the shoe up with a strap or rope. Figure 3: Docklands Light Railway train with 3rd rail bottom contact electrification system. An example of a bottom contact shoe as used on the Dockland Light Railway line in London is shown in Figure 3 and in the video (Figure 5). Some top contact systems have also used spring loading but they are mechanically more difficult to control because of the hunting action of the bogie and the risk that the shoes will get trapped under the head of the rail and turn it over. It can use either DC (direct current) or AC (alternating current), the former being, for many years, simpler for railway traction purposes, the latter being better over long distances and cheaper to install but, until recently, more complicated to control at train level.

Both systems are provided to allow train testing from the main line or London Underground. London Underground is the largest user of the 4-rail system in the world. The 3rd rail system is common around the world but the 4th rail is rare. AC systems always use overhead wires, DC can use either an overhead wire or a third rail; both are common. The most common reason is when a shoe breaks off and its connecting lead to the electrical equipment on the train has to be secured safely. Both overhead systems require at least one collector attached to the train so it can always be in contact with the power. Being the simplest, it has drawbacks, not the least of which is that it is exposed to anyone or any thing which might come into contact with it. Side contact is not much better than top contact but at least it is less exposed. It also suffers during bad weather, the smallest amount of ice or snow rendering top contact third rail systems almost unworkable unless expensive remedies are carried out. It is a top contact system. The third rail system uses a "shoe" to collect the current on the train, perhaps because it was first called a "slipper" by the pioneers of the industry (it slipped along the rail, OK?) but it was not very pretty to look at, so perhaps someone thought shoe was a better description.

Figure 4: 3rd rail current collection system on the New York Subway showing the third rail with a wooden cover fitted to reduce the effects of snow and ice. Figure 6: Diagram showing a 3rd rail DC power supply system and how current rail gaps are provided where the substations feed the line. Figure 1: A section of the Old Dalby test track in England showing both third rail and overhead electrification. Transmission of power is always along the track by means of an overhead wire or at ground level, using an extra, third rail laid close to the running rails. Coaxial design helps to further reduce low-frequency magnetic transmission and pickup. This also helps in drastic lowering of electromagnetic radiation. Its steel armoring helps it to defy any mechanical extremities that can harm the cable. Bottom contact is best - you can cover effectively most of the rail and it is protected from the worst of the cold weather. Later systems had radially mounted shoes to provide more stable contact through lever action. Woe betide the driver who stops his train with all the shoes "off juice" or "gapped". Since the current may have been switched off to stop an arc or because of a short circuit, it is important that the train does not connect the dead section to the live section by passing over the gap and allowing its busline to bridge the gap.

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