This is something I've been quietly tracking for a while actually. The Glastonbury area doesn't have a lot of railroad infrastructure but I did map out some Dogman adjacent reports from the wider Somerset region last year and there was a noticeable clustering near old disused lines and rail embankments specifically.
My working theory is that it's less about the rails themselves and more about the corridor they create. Old railway lines cut through terrain in very deliberate straight lines, they disrupt natural boundaries, they create long thin strips of overgrown vegetation and they often run through areas that haven't seen serious human activity in decades. If something like this exists and is territorial, those corridors would be ideal movement routes well away from populated areas.
I've also noticed in American reports that a lot of the sightings near rail lines happen at night near bridges or tunnels specifically. Would be interested to know if anyone here has mapped the UK reports in any systematic way. Has anyone cross referenced ordnance survey data with the sighting locations?