The Tram-Train can just be on the surface or become part of a subway line. https://en.wikipedia.org/wiki/Tram-train#Technology
The Muni_Metro in SF is a good example. https://en.wikipedia.org/wiki/Muni_Metro#Market_Street_subway
https://en.wikipedia.org/wiki/Muni_Metro#Future
https://www.nycsubway.org/wiki/Countries_Around_the_World
https://therabbitportal.blogspot.com/search?q=metro+and+subway
