>>1874472You could probably do this with Glasgow, it has one of the largest commuter rail networks outside of London and there have been attempts in the 80s to improve connections with the subway.
https://www.youtube.com/watch?v=UQnCy82yaVE>>1874809Many of Glasgow's commuter rail lines are isolated from the rest of the network; the only trains that run on the Cathcart Circle, Inverclyde, and Paisley Canal lines are local trains, and where intercity trains do run, they never run through the entirety of a line (except for the sleeper through Queen Street low level).
In London, the Metropolitan Line shares its tracks with the London-Aylesbury line. The Bakerloo Line shares its tracks with the Watford DC line. The District Line shares its tracks with the North London Line.
The latter two examples are run by the Overground (which is still National Rail, and TfL wasn't always the operator), but the point is that you don't need fully isolated networks to focus on local connectivity.
