Oooooh, a new trains thread
I think what pushed London into the tube was the prohibition on above ground railways through central London area; only 1 railway (the now Thameslink route) managed to work through this. London is widespread enough that people needed onwards transit from their terminal. It widely used the tactic of tunnelling in the centre, and then taking over suburban main line routes as their outer reaches.
Other areas I think could manage this:
- Liverpool; essentially as Merseyrail ended up, although the Wirral Lines could head further east and take over some of the Outer Rail Loop.
- Manchester; north/south through the centre to connect the separate Victoria and Piccadilly stations and the city centre, taking over the suburban routes to the north and south of the city.
- Leeds/Bradford; again a short city centre tunnel, and take over some of the suburban routes.
All are "industrial" cities; large amounts of people likely to use public transport, cities of a decent size and have plenty of suburban rail to use.