Interesting thread this. I've not had a direct Dogman encounter myself but the railroad connection keeps cropping up and it's hard to ignore at this point.
My thinking is it probably ties back to ley lines more than the railways themselves. A lot of the old Victorian rail routes were laid out following natural landscape features - valleys, ridgelines, that sort of thing - and those corridors often overlap with older energy pathways. The engineers didn't know what they were doing but they were essentially following the same routes our ancestors marked out centuries before.
There's a stretch of disused track about 12 miles from me near the edge of Cranborne Chase and locals have reported strange things out there for years, long before the Dogman label became popular. Whether it's the same phenomenon is anyone's guess.
Would be curious if anyone has mapped the sighting clusters against actual ley line surveys rather than just the rail routes. That might tell us something more useful.