The Scotrail website says on their route maps page:
Taking the train is one of the best ways to enjoy our country's stunning landscapes, with some places only accessible by rail.
A random selection of a dozen or so stations from the map didn't turn anything up, so I'm curious if anyone knows which "places" they're referring to.
Starting a list of candidates as Community Wiki, which people may be able to confirm or refute with further research. Feel free to add to this!
Corrour station is "accessible by rail only" in the sense that there is no public road access (but access via a private road is possible). (Credit B. Liu)
Altnabreac is described by a 2025 BBC article as "10 miles from the nearest road", though there do appear to be unpaved roads or tracks reaching it. Wikipedia says it is on a private dirt road, and is 6.8 miles from the nearest paved road.