Yeah this has been on my radar for a while actually. There's something worth digging into here beyond just coincidence. Old rail corridors cut through massive stretches of wilderness that were basically untouched before they were laid down, and a lot of those routes followed older indigenous pathways and animal migration trails going back centuries. So you've already got locations with a deep history of unusual activity before the railways ever showed up.
The Sheffield area has a few old disused lines and I've spoken to people who've had encounters or heard things in those corridors at night that they couldn't explain. Whether that's Dogman specifically I can't say but the pattern of "weird stuff near old rail lines" is definitely a recurring theme when you start cataloguing regional reports properly.
My working theory is it's less about the railways themselves and more about what the railways were built on top of. Would be interested to know which specific lines people are seeing this clustering around.