You don't need separate stations, just enough separate tracks. Berlin Hbf for example has its ICE trains mainly arrive in the basement level, with regional and

You don't need separate stations, just enough separate tracks. Berlin Hbf for example has its ICE trains mainly arrive in the basement level, with regional and local trains on physically higher levels (worth seeing for anyone visiting). But most large cities' main stations have 10+ parallel tracks, usually with a large junction a km or so each side to let trains funnel into the correct platform (see e.g. Hannover: https://www.openstreetmap.org/#map=14/52.3813/9.7435 https://www.openstreetmap.org/#map=14/52.3813/9.7435). Plenty of room for local, regional and ICE trains to be all together.