It is maybe 6-8 lines total and was incredibly useful because it was designed to be, I didnt need a car to get anywhere and it was clean and cheap (2 euros per trip) .
The new Q-line in Detroit is 1 line that is three miles total and is more designed as a tourist trap, most people dont work three miles away from where they live. You need to connect it to other systems like an airport or a bus station, and you need more than 1 line (duh).
If you gave everyone horses and only build a three mile road you wouldn't say that "cars are a failure, and roads lose money". For some reason in the States we build out a tiny one line system and then throw our hands up in the air when nobody rides it.