This is something I've been quietly tracking for a while actually. The correlation keeps showing up in North American reports and I cant quite shake it. Old rail lines often follow natural geographical corridors - river valleys, ridge lines, gaps through dense woodland - so they may just be mapping onto existing wildlife movement routes that these things use anyway.
There's also the abandoned infrastructure angle. Disused rail tunnels and culverts would make excellent denning spots for something large that wants to avoid human contact. The Herefordshire/Welsh border has similar patterns with old drove roads and I've noticed local "big cat" and anomalous canid reports cluster along them fairly consistently.
What I'd want to know is whether the sightings are happening near active lines or specifically derelict ones. That distinction would tell us a lot about whether it's a habitat thing or something weirder - like whatever gets reported around liminal or transitional spaces more broadly. Would be good to hear what others have mapped out on this.