https://en.wikipedia.org/wiki/Rapid_transit#Lines
https://en.wikipedia.org/wiki/Rapid_transit_in_Germany ,
https://www.youtube.com/watch?v=4194-0Erq-I ,
https://www.youtube.com/watch?v=79MC2gJ4_2Y
https://en.wikipedia.org/wiki/Urban_rail_transit#Urban_rail_transit_types