Yeah this has caught my attention too actually. Suffolk has a fair few old railway lines, some of them closed since Beeching in the 60s, and the Black Shuck reports around here have always clustered near the old trackbeds and routes. Whether thats dogman proper or something else is debatable but the pattern is real.
My thinking is that old railways cut through landscape in very specific ways - cuttings, embankments, long straight corridors through woodland. They basically create ley-line style routes through the terrain. If something large and territorial is using them as travel corridors that would explain the clustering.
Interested whether anyone has tried overlaying old OS maps showing disused lines against reported sightings. I've done a rough version of this for East Anglia and the correlation is honestly striking. Would love to compare notes with anyone doing similar work elsewhere in the country.