Never looked into Dogman specifically but the rail line thing catches my attention because of ley lines. Old rail routes in Britain especially often follow ancient trackways and ley alignments, sometimes deliberately, sometimes just because the land dictates it. I've done a fair bit of walking the North Downs here in Kent following suspected leys and the old drove roads and rail cuts through the same valleys repeatedly.
Whether thats attracting something or just where people are moving through and spotting things, hard to say. But liminal spaces along transport corridors do seem to generate more reports generally, not just cryptid stuff, ghost sightings, UFOs, the lot.
Anyone mapped the specific sightings against OS maps? Would be interesting to see if theres a pattern beyond just "near railway."