Editing Light rail (section)

===Light rail operating on mainline railways===
{{Main|Tram train}}
[[File:Cappel Stadtbahn02 2005-12-30.jpg|right|thumb|On the [[Karlsruhe Stadtbahn]], trams sometimes share mainline tracks with heavy rail trains.]]
[[File:Carling O-Train Station.jpg|thumb|The [[Line 2 (O-Train)|Trillium Line]] in [[Ottawa]] was built along a freight railway and is still occasionally used by freight traffic overnight.]]
Around [[Karlsruhe]], [[Kassel]], and [[Saarbrücken]] in Germany, dual-voltage light rail trains partly use mainline railroad tracks, sharing these tracks with heavy rail trains. In the [[Netherlands]], this concept was first applied on the [[RijnGouweLijn]]. This allows commuters to ride directly into the city center, rather than taking a mainline train only as far as a central station and then having to change to a tram. In France, similar [[tram-train]]s are planned for Paris, [[Mulhouse]], and [[Strasbourg]]; further projects exist. In some cases, tram trains use previously abandoned or lightly used heavy rail lines in addition to or instead of still in use mainline tracks. In 2022, Spain opened the [[Cádiz Bay tram-train|Cádiz TramBahia]], where trams share track with [[Cercanías Cádiz|commuter]] and long-distance trains from the main terminus in the city and curve off to serve cities without a railway connection.

Some of the issues involved in such schemes are:
* compatibility of the safety systems
* power supply of the track to the power used by the vehicles (frequently different voltages, rarely third rail vs overhead wires)
* width of the vehicles to the position of the [[railway platform|platforms]]
* height of the platforms

There is a history of what would now be considered light rail vehicles operating on heavy rail [[rapid transit]] tracks in the US, especially in the case of [[interurban streetcar]]s. Notable examples are [[Lehigh Valley Transit]] trains running on the [[Philadelphia and Western Railroad]] high-speed third rail line (now the [[Norristown High-Speed Line]]). Such arrangements are almost impossible now, due to the [[Federal Railroad Administration]] refusing (for crash safety reasons) to allow non-FRA compliant railcars (i.e., subway and light rail vehicles) to run on the same tracks at the same times as compliant railcars, which includes locomotives and standard railroad passenger and freight equipment. Notable exceptions in the US (known as [[hybrid rail]]) are the [[NJ Transit]] [[River Line (New Jersey Transit)|River Line]] from [[Camden, New Jersey|Camden]] to [[Trenton, New Jersey|Trenton]] and Austin's [[Capital MetroRail]], which have received exemptions to the provision that light rail operations occur only during daytime hours and [[Bordentown Secondary#Today|Conrail]] freight service only at night, with several hours separating one operation from the other. The [[O-Train]] [[Trillium Line]] in Ottawa also has freight service at certain hours.