page_title: Railway electrification system
section_title: Third rail
hierarchical_section_title: Railway electrification system / Direct current / Third rail
caption_attribution_description: With surface contact third (and fourth) rail systems a heavy "shoe" which is suspended from a wooden beam attached to the bogies (wheel units) collects power by sliding over the top surface of the electric rails. This view shows a class 313 train which operates on Silverlink and WAGN routes primarily to the north and west of London. Photograph taken by SPSmiler.
context_page_description: A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply.
  Electric railways use either electric locomotives, electric multiple units or both.
  Electricity is typically generated in large and relatively efficient generating stations, transmitted to the railway network and distributed to the trains. Some electric railways have their own dedicated generating stations and transmission lines, but most purchase power from an electric utility. The railway usually provides its own distribution lines, switches, and transformers.
  Power is supplied to moving trains with a continuous conductor running along the track that usually takes one of two forms: an overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings, or a third rail mounted at track level and contacted by a sliding "pickup shoe". Both overhead wire and third-rail systems usually use the running rails as the return conductor, but some systems use a separate fourth rail for this purpose.
context_section_description: Most electrification systems use overhead wires, but third rail is an option up to 1,500 V, as is the case with Shenzhen Metro Line 3. Third rail systems exclusively use DC distribution. The use of AC is not feasible because the dimensions of a third rail are physically very large compared with the skin depth that the alternating current penetrates to 0.3 millimetres or 0.012 inches in a steel rail. This effect makes the resistance per unit length unacceptably high compared with the use of DC. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.
caption: With top-contact third (and fourth) rail a heavy shoe attached to the underside of a wooden beam which in turn is attached to the bogie, collects power by sliding over the top surface of the conductor rail.
exif: {}