Yes, it's originally about Salford (but so was Ray Davies' Waterloo Sunset!!),

Hmmm, I didn't know there was an underground railway in Salford :-)

but there's really nothing in the song that pinpoints it to one place. Another good example is Paul Simon's 'Homeward Bound', written whilst waiting for a train on Widnes station during a tour of UK folk clubs.

I don't think Simon said anything in the song to say he was at Widnes - he could have been on any railway station, anywhere, at least in the western world at the time. When I first heard it I imagined him being in a US rail station, before I later read he had been on tour in the UK when he wrote it.

As for Waterloo Sunset, that is clearly about Waterloo in London. Although the sentiments expressed in the lyrics could apply to lots of other places, the lyrics refer explicitly to one location. Dirty Old Town and Homeward Bound don't.

I don't think a song has to be 'generic' in terms of location for people to be able to identify with it. I'm sure people who've never been to London could identify with Waterloo Sunset, even though it is more or less 'pinpointed' to that place.