Genuinely hadn't thought about this until you posted it but yeah, there does seem to be a pattern there. Old rail corridors often run through pretty isolated terrain, cut through woodland, follow river valleys - basically threading through the kind of places humans don't spend much time anymore. The infrastructure gets abandoned but the land either side stays wild.
Not a Dogman researcher specifically but I follow the reports and the concentration along old lines in the Midwest US does stand out. Over here in the UK I've noticed similar clustering of big cat and anomalous creature sightings along old canal routes and disused railway cuttings, which might point to something about linear landscape features generally rather than rails specifically.
My working theory is it's less about the tracks themselves and more about the undisturbed corridors they create. Wildlife uses them as travel routes and if something larger is out there, it probably does the same.
Has anyone mapped these properly? Like actually plotted sighting coordinates against old rail network maps? That kind of overlay could tell us something useful rather than just noticing the pattern anecdotally.