They avoided the naming problem in hannover by calling it Stadtbahn (city rail).
That isn't avoiding the problem. "Stadtbahn" is the correct terminology in academic traffic theory.
German cities, including Stuttgart, Hannover, and Frankfurt am Main, have U-Bahn system which runs partially on-street sections and uses high floor vehilces. In Germany, U-Bahn is usually formed as a result of upgrade of existing tram network. If I remember correctly, there are only 4 cities (Berlin, Hamburg, Muenchen, and Nuernberg) which have U-Bahn system which were originally built as a separate U-Bahn system.
In germany academic theory and law divide into three kinds of system.
- "U-Bahn" / "Hochbahn" (underground / elevated metro) meaning all crossings with streets or other rail systems are level-free and tracks are completely separated from street/car traffic.
- "Stadtbahn" meaning tracks are completely separated from street/car traffic but can have level-crossings with streets.
- "Straßenbahn" / "Tram" meaning tracks don't need to be separated from street/car traffic and thus have crossings in same level with streets.
Most important point is, that they all are run under the terms of "BOStrab" ("
Straßenbahn-Bau- und Betriebsordnung", "regularities for building and operation of trams"). The first and second layout are usually both named "U-Bahn" despite their unequality in academic theory. So according to german law underground would be considered as "tram".
In germany every kind of transport system that is not a ropeway or conventional railway is run unter the terms of BOStrab, which would include light railway, underground, tram and so on. Basic point is the length of vehicles when running on streets. Those cars must not be longer than 75 meters. Cars with seperated tracks do not have this limitation. But I think - to think in-game - simutrans is not capable of controlling this difference.
The highth of the floor in vehicles is not important. Most U-Bahn trains are high-floor vehicles whereas Tram trains are usually low-level vehicles. But that's not the point to group them. Even the gauge of tracks is insignificant. For example in Frankfurt Stadtbahn and Tram have 1435 mm gauge. In Darmstadt Tram has only 1000 mm gauge. But trams in both cities are nowadays run with low-floor vehicles and were run with high-level vehicles in the past.
In the system of heavy rail there is also something similar to "metro" in germany. It is called "S-Bahn". Best equivalent in english would be local commuter railway I think. Mostly S-Bahn trains run more far into the suburbs than U-Bahn / Stadtbahn / Tram trains. Main difference in Germany is, that U-Bahn / Stadtbahn / Tram systems are run by the cities whereas S-Bahn is generally run by Deutsche Bahn.
Back to simutrans I would consider underground / metro to be both, tram and train. It depends wheter you want to mix tram and metro (then it's tram) or metro and heavy rail (then it's train). For example the "London Underground" would be train. The DLR (to stay in London) would be tram.
In Germany S-Bahn would be train, U-Bahn would be tram even if they have the same gauge of tracks and nearly same width of vehicles.
So thinking in-game I would appreciate if there were tunnels for tram / metro (as tram) in simutrans with faster tracks (could easily be 100 km/h top-speed) and larger vehicles. And thinking about gates for tunnels I find the 130-km/h-tunnels of 128.japan very stylish and useful for that purpose.
