Conventional wisdom is that trams are those bus-like things that run on rails in the street and trains are those long things that run on grade-separated tracks to big stations, and never the twain shall meet. However, the new Sheffield to Rotherham tram-train is rubbing away at that distinction. Based on an idea pioneered in Karlsruhe, Germany, the tram-train runs on conventional tramlines in the city centre, then transferring seamlessly to heavy rail lines to go further afield. It's not the first time trams and trains have shared tracks in the UK. Prior to 1967, freight destined for Fairfield's shipyard in Goven used the tracks of Glasgow corporation tramway to access the shipyard. However, there are technical barriers to allowing tramcars onto the railway. Vehicles need to meet crashworthiness standards and be compatible with signalling on the "main line". Another issue is power supply. A tram-train is proposed to connect to Glasgow Airport, but the main line between Glasgow and Paisley is electrified to 25kv AC, while street running trams are limited to 750v DC power, meaning tramcars would need dual-voltage electrical systems. Another potential tram-train route is in Edinburgh, where re-opening of the south suburban line is currently blocked by congestion at Edinburgh Waverley station, but a tram-train could bypass the station by transferring onto the Edinburgh tram lines.
In British railway history, there have been a few lines that have blurred the distinction between tramways and railways, such as the Swansea and Mumbles railway in Wales. Now a new generation of tram-trains is set to continue breaking boundaries.