A tram-train is a light-rail public transport system where trams run through from an urban tramway network to main-line railway lines which are shared with conventional trains. This combines the tram's flexibility and accessibility with a train's greater speed, and bridges the distance between a main railway stations and a city centre.
There is also a train-tram, which is a train modified to also run on tramlines. Generally, the tram-train and train-tram are interchangeable, although a train-tram is based on a train design modified to also run as a tram and a tram-train is based on a tram design modified to also run on a train line.
The tram-train concept was pioneered with the Karlsruhe model in Germany, and has since been adopted on projects such as the RijnGouweLijn in the Netherlands, at Mulhouse in France and in Kassel and Saarbrücken in Germany.
Tram-train vehicles are dual-equipped to suit the needs of both tram and train operating modes, with support for multiple electrification voltages if required and safety equipment such as train stops and other railway signalling equipment. The Karlsruhe and Saarbrücken systems use ‘PZB’ or ‘Indusi’ automatic train protection, so that if the driver passes a signal at stop the emergency brakes are applied.
The idea is not new: in the early 20th century, interurban streetcar lines often operated on the same tracks as steam trains, until crash standards prevented track sharing. In 1924, in Hobart, Tasmania, sharing of tracks between tram and train was proposed.
The difference between modern tram-trains and the older interurbans and radial railways is that tram-trains are built to meet mainline railway standards, rather than ignoring them. An exception is the USA's River Line in New Jersey which runs along freight tracks with time separation: passenger trains run by day, and freight by night.
- Alicante Tram, Alicante, Spain
- Chemnitz, Germany - 750 V DC
- Kassel, Germany (2006)
- Nordhausen, Germany - 600 V DC/on-board diesel engine
- Nantes, France (Nantes-Clisson, SNCF) light train rolling stock and not operating in urban section.
- Paris, France (Line T4, SNCF) light train rolling stock and not operating in urban section.
- Mulhouse tramway, Mulhouse, France
- Saarbrücken, Germany
- Stadtbahn Karlsruhe, Karlsruhe, Germany - 750 V DC/15 kV AC
- Sassari, Italy
- Cagliari, Italy
- RandstadRail, The Hague, Rotterdam and Zoetermeer, Netherlands
- Zwickau, Germany - on-board diesel engine (Light-weight RegioSprinter diesel railbuses that also operate on street tramway)
- Capital MetroRail in Austin, Texas, USA - commuter rail that shares more commonality with tram-train operation, with downtown street running and usage of mainline track. Uses diesel multiple units.
- River Line (New Jersey, United States) - diesel multiple units using main line tracks between Trenton, New Jersey and Camden, New Jersey in a time-sharing agreement with the freight companies.
- The O-Train Trillium Line in Ottawa, Canada which uses European-style main line trains, and the Newark City Subway extension in Belleville and Bloomfield, New Jersey (with an FRA-imposed time-share waiver) do not qualify it as a tram-train per se, whose chief characteristic is shared-use of main-line tracks at all times.
- UTA TRAX uses former Denver and Rio Grande tracks as well as street trackage to service Salt Lake City. Between the hours of midnight and six A.M., Union Pacific freight trains use much of the trackage, up to just past 2500 S to service a number of industries along the line.
- Aarhus Letbane, Denmark (under construction, to be opened 2016)
- Braunschweig, Germany
- Bratislava, Slovakia
- Grenoble, France
- Groningen, Netherlands
- île de France (Paris region), France. The system is called Tram Express by the transport authority STIF: 3 lines are scheduled (Nord, Ouest, Sud). The light train rolling stock will only roll on national rail network in northern line (Tram Express Nord). In the Western line (Tram Express Ouest) a short section of 3.6 km (2.2 mi) is an urban tram section of the 19 km (11.8 mi) line. The southern line (Tram Express Sud) is a 20 km (12.4 mi) line, 10 km will be tram section and the 10 km another will roll on national rail network.
- Kiel, Germany
- Košice, Slovakia (under construction, to be opened in 2020)
- León, Spain
- Liberec — Jablonec nad Nisou, Czech Republic
- Linköping, Sweden
- Lyon, France
- Manresa, Catalonia, Spain
- Metro Mondego, Coimbra, Portugal
- Midland Metro extensions in the West Midlands conurbation, England
- Porto Metro Lines B and C, Porto, Portugal
- RijnGouweLijn, Netherlands
- Metro de Sevilla. Seville has one metro line and one tram line that are not connected, but the long-term intention is to link the metro and tram systems.
- Strasbourg, France
- Szeged, Hungary
- TramCamp, Camp de Tarragona, Catalonia, Spain
- Wrocław, Poland (2005) — 600 V DC/3 kV DC
- Riga, Latvia
- Tampere, Finland
- Turku, Finland
In March 2008 the UK Department for Transport released details of a plan to trial diesel tram-trains on the Penistone Line for two years starting in 2010. There was no commitment to connect them to the Sheffield tram network, and in September 2009 the idea was withdrawn as it was deemed not economically viable for a trial due to the cost of the extra development required for the diesel engines to meet the forthcoming stringent EU emission regulations. Instead single-voltage electric tram-trains will be trialled between Rotherham and Sheffield.
A tram-train trial in the Manchester area was ruled out as the Department for Transport wanted to try low-floor tram-trains, whereas Manchester Metrolink cars have high floors.
In August 2009 the Liverpool Daily Post reported that a new Merseyrail tram-train link to Liverpool John Lennon Airport was under consideration. The Merseyrail Northern Line and the City Line between Liverpool Lime Street and Liverpool South Parkway were being assessed. From South Parkway the tram-trains would transfer seamlessly to a new tramway. A link from Edge Hill in the east of the city to the Arena at Kings Dock near the city centre was also being considered.
Other suggested schemes for England & Wales include:
- Stourbridge — Dudley — Walsall
- Leeds — Harrogate — York
- Leeds — Wakefield
- Leeds — Bradford Interchange — Bradford
- Leicester — East Midlands Parkway — East Midlands Airport
- Cardiff — Caerphilly — Treharris - Quakers Yard- Pontypridd-Cardiff
- Coryton — Cardiff City Centre — Roath Basin
- Leeds — Selby
- Blackpool — Lytham St Annes
- Manchester — Marple 
- Manchester — Northwich (via the direct Manchester Metrolink line to Altrincham)
- Norwich — Rackheath
- Tees Valley Metro (later phases)
- Abbey Line (Watford Junction to St Albans Abbey)
- Thanet Light Rapid Transit, Kent
- Medway Metro / MedRail, Kent — project now "mothballed"
- Walsall — Wolverhampton
- Adelaide, South Australia – On June 5, 2008, the Government of South Australia announced plans for train-tram operation on the Adelaide Metro's Outer Harbor/Grange train lines and City West-Glenelg tramline extension as part of a 10-year A$2 billion public transport upgrade.
Models of tram designed for tram-train operation include:
- Alstom’s Dualis, derived from the Citadis
- Bombardier’s Flexity Link
- Bombardier-Adtranz A32.
- Siemens' Avanto (also called S70)
- Vossloh Citylink NET
- Karlsruhe model
- Light rail
- rail profile - light and heavy rail vehicles do not necessarily have the same rail and wheel profiles.
- Grooved rail
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- TramTrain - the 2nd generation: Searching for the ‘ideal’ TramTrain-city
- New TramTrain for Mulhouse - Reportage and images (english/german.)
- Construction of the TramTrain system in Mulhouse with images (english/german.)
- tram-train of Karlsruhe transformed in a subway in the center
- Leeds City Region proposal
- (Jane's) Urban Transit Systems