Word! You speak out of my soul.
I've always missed a London Underground, never thought of a NY subway, but would be awesome.
The biggest problems are the asssets, as you said. Especially the Tunnels.
Two ways: build the tracks overground and enclose them or make custom tunnellofts.
I think the second way is better although it is more difficult.
One would need for examample a loft with huge space around the tracks to make a station.
Complicated, but feasible.
Third rails are already possible (Isle of Wight). Custom signals, trains and other things like signs are also needed. A big negative aspect is the lack of working lights, but I think this is fixed till the end of the year.
So when do you start?
