Been mulling this over for a while actually. Up here in Scotland we don't get the Dogman reports the same way the US does but I've been following the American cases closely and the railroad correlation is genuinely striking once you start mapping them out.
My theory for what its worth - old rail lines often cut through wilderness that was left largely untouched once the lines closed. No development, no foot traffic for decades sometimes. Perfect territory for something to establish itself. Also the old cuttings and embankments create natural corridors through terrain, something using them to move around wouldn't be daft at all.
There's also the earth mysteries angle which is more my usual territory. Rail lines were often laid following older paths and trackways, sometimes ley lines if you're that way inclined. Wouldn't be the first time high strangeness clusters around those old routes.
Anyone done proper mapping on this? Would love to see the data overlaid against abandoned vs active lines, might tell us something useful.