Metro/Tram-Trains are Multi-Talents or Chameleons, that operate locally on the City Rail Networks and as well inter-regional or between cities on Main-Line Railway networks, bringing commuters from one City Center to the next City Center without changing the transport mode. Metro/Tram-Trains operate seamless with one-and-the-same Rail Vehicle on Lines of Metros, Subways, Street Cars, Street Trams, suburban Commuter Rail, Intercity Regional Rapid Transits and Main Line Railways. The system should be unigaue, otherwise dual-gauge or gauntlet tracks are needed. Metro/Tram-Trains  combine Urban Rail with Regional Rail Transport.

The most advanced modern Tram-Train system in the world operates currently in the Karlsruhe Region, Germany, on a rail network of 533 km, using 354 km railway lines together with the infrastructures of the German Federal Railway. Further modern Tram-Train Networks can be found in the regions of Cologne, Bonn, Saarbrücken, Stuttgart (Neckar-Alb), Chemnitz, Nordhorn in Germany, Cadiz in Spain, Mulhouse and  Nantes in France, Linz and Salzburg in Austria, Copenhagen in Denmark and Szeged in Hungary. Cases for combined Metro and Railway systems in India are the Nagpur and Dehradun-Rishikesh-Haridwar Regions.

Modern Thyristor Technology enables nowadays the seamless operation under different electric feeding systems: 650/750/1000 V DC, 15 kV, 16 2/3 Hz AC and 25 kV, 50 Hz AC.

For a combined and intermodal City-Tram/Metro and Regional Railway operation the track gauge should be the same, either both systems in Meter Gauge, Standard Gauge or Broad Gauge.

With the introduction of Metro Rail and High-Speed Rail operating on Standard Gauge, India had to leave its Unigauge Strategy with Broad-Gauge only. The history and dilemma of Indian Broad Gauge is discussed in the PDF below,  for free download COMBINED TRAM METRO TRAIN AND RAILWAY TRAINS