The tram-train concept derives from the intention to integrate suburban railways (pre-existing lines which connects the urban and the districts areas) and tramway lines (running inside the urban perimeter).
This requires common features for trams and local trains, first of all the same gauge; then, it's necessary to have dual voltage vehicles, able to run in tramway mode (normally with 600/750 V DC overhead power supply) or in train mode (often 15/25 KV AC if the line is electrified, else with diesel power supply).
Karlsruhe Stadbahn is an emblematic case of properly said tram-train, whit trams equipped with on-board inverter in order to switch from tramway mode (DC) to train mode (AC) before running along the railway tracks (in track sharing with suburnan and regional trains).
Saarbrucken tram-train system has similar characteristics (even if the whole net has a minor extension), with dual vehicles in track-sharing with ordinary trains.
Kassel system (as well in Germany) shows an interesting feature: the tram rolling stock is able to run along the not-electrified railway tracks converting from DC voltage to diesel power supply.
In some other cases, the tramway net re-use dismissed or under utilized railway tracks (like Manchester); the track sharing is minimized and the tramway subjects the train infrastructures to its service.
The pure tram-train concept is much far when the tramway system simply runs over elder railways tracks (sometimes partly demolishing and rebuilding some of the existing infrastructures); in these cases (like Alicante) the dual voltage it's a simple way to reduce system total costs re-using existing tracks and plants.
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