A tram-train is a type of light rail vehicle that meets the standards of a light rail system (usually an urban street running tramway), but which also meets national mainline standards permitting operation alongside mainline trains. This allows services that can utilise both existing urban light rail systems and mainline railway networks and stations. It combines the urban accessibility of a tram or light rail with a mainline train's greater speed in the suburbs. The modern tram-train concept was pioneered by the German city of Karlsruhe in the late 1980s, resulting in the creation of the Karlsruhe Stadtbahn. This concept is often referred to as the Karlsruhe model, and it has since been adopted in other cities such as Mulhouse in France and in Kassel, Nordhausen and Saarbrücken in Germany. An inversion of the concept is a train-tram; a mainline train adapted to run on-street in an urban tramway, also known as the Zwickau Model. Technology The tram-train often is a type of interurban — that is, they link separate towns or cities, according to George W. Hilton and John F. Due's definition. Most tram-trains are standard gauge, which facilitates sharing track with main-line trains. Exceptions include Alicante Tram and Nordhausen, which are metre gauge. 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. History The idea is not new; in the early 20th century, interurban streetcar lines often operated on dedicated rights-of-way between towns, while running on street trackage in town. The first interurban to emerge in the United States was the Newark and Granville Street Railway in Ohio, which opened in 1889. In 1924, in Hobart, Australia, sharing of tracks between trams and trains was proposed. The difference between modern tram-trains and the older interurban and radial railways is that tram-trains are built to meet mainline railway standards, rather than ignoring them. An exception is the United States' River Line in New Jersey, which runs along freight tracks with time separation; passenger trains run by day, and freight by night. Existing systems Asia Japan Fukui: Fukui Fukubu Line Europe Austria Gmunden: Traunsee Tram (2018) Vienna: