Been lurking on this topic for a while and honestly it's something I've been quietly tracking myself up here in Cumbria. We've got loads of old disused railway lines - a fair few left over from the Beeching cuts in the 60s - and I've come across a couple of accounts locally that do seem to cluster around those old trackbeds.
My thinking, for what it's worth, is that the old railway corridors essentially became natural wildlife highways once the trains stopped running. Overgrown, undisturbed, connecting remote areas to each other. If something large and unusual is moving around, those routes make a kind of practical sense, don't they? Low human traffic, good cover, linear routes between woodland areas.
What I'm less sure about is whether this is genuinely significant or just confirmation bias - we notice the railway connection when it's there and ignore the dozen sightings that happen nowhere near one.
A few questions I'd throw out to the group:
Are we talking active lines or specifically disused ones?, Has anyone cross-referenced sighting databases like the NAWAC reports against OS maps with old railway routes marked?, Is there any pattern with time of year on top of the location correlation?
I'm only a beginner at this sort of research so I'm probably missing obvious methodology here. But it does feel like there's something worth digging into properly rather than just collecting anecdotes. Would be curious whether anyone from the States has noticed similar patterns given how many abandoned rail corridors you've got over there.