Editing Tram (section)

===Power supply===
{{main|Railway electrification|Current collector|List of tram systems by gauge and electrification}}
[[File:Tramway graz03.jpg|thumb|[[Overhead line]]s are used for most tram and light rail systems.]]

Electric trams use various devices to collect power from [[overhead line]]s. The most common device is the [[Pantograph (transport)|pantograph]], while some older systems use [[trolley pole]]s or [[bow collector]]s. [[Ground-level power supply]] has become a more recent innovation. Another technology uses [[supercapacitor]]s; when an [[insulator (electricity)|insulator]] at a track switch cuts off power from the tram for a short distance along the line, the tram can use energy stored in a large [[capacitor]] to drive the tram past the gap in the power feed.<ref>{{cite web |url=http://www.facstaff.bucknell.edu/mastascu/elessonshtml/lc/capac1.htm |title=An Introduction To Capacitors |publisher=Facstaff.bucknell.edu |access-date=8 March 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150309023326/http://www.facstaff.bucknell.edu/mastascu/elessonshtml/LC/Capac1.htm |archive-date=9 March 2015}}</ref>
The old tram systems in London, [[Manhattan]] (New York City), and Washington, D.C., used live rails, like those on third-rail electrified railways, but in a conduit underneath the road, from which they drew power through a [[plough]]. It was called [[conduit current collection]]. Washington's was the last of these to close, in 1962. No commercial tramway uses this system anymore. More recently, an equivalent to these systems has been developed which allows for the safe installation of a [[third rail]] on city streets, known as surface current collection or [[ground-level power supply]]; the main example of this is the new tramway in [[Bordeaux]].

====Ground-level power supply====
{{Main|Ground-level power supply}}
[[File:Bordeaux-aps+isolation&joint.jpg|thumb|A section of [[Ground-level power supply|APS]] track in [[Bordeaux]] with powered and neutral sections.]]

A ground-level power supply system, also called surface current collection or {{Lang|fr|alimentation par le sol}} (APS), is an updated version of the original stud type system. APS uses a third rail placed between the running rails, divided electrically into eight-metre powered segments with three-metre neutral sections between. Each tram has two power collection skates, next to which are antennas that send radio signals to energize the power rail segments as the tram passes over them.

Older systems required mechanical switching systems which were susceptible to environmental problems. At any one time no more than two consecutive segments under the tram should be live. Wireless and solid state switching eliminate mechanical problems.

[[Alstom]] developed the system primarily to avoid intrusive power supply cables in the sensitive area of the old city of old [[Bordeaux]].<ref>[http://www.railway-technology.com/features/feature1096/ Allez le Tram from Railway-Technology.com]. Retrieved 15 February 2009.</ref>