I regularly take my dog out to heritage railways. She loves the fuss and attention from staff and other passengers. It's also good for her to get out and about and be exposed to different sounds and smells. On our most recent visit to the SVR, she got incredibly excited when we got off the main line train at Kidderminster and she realised where we were going! The only railways we've come across that don't allow dogs are Snowdon Mountain Railway (understandable given the space constraints) and Bressingham. Churnet Valley seems to tolerate rather than welcome dogs, charging £10 when most railways have a nominal fee or are free. TripAdvisor suggests it's an over-reaction after a bad experience. We don't take her into dining areas and if we end up sharing a compartment/table we ask the other occupants if they are OK with dogs first. I'd hope all other responsible owners do the same.