It's completely irrelevant in my opinion. Whether someone is a railway enthusiast or not is not an indicator of whether they are suitable for any particular role.
I would not mention it at an interview. Not because I think it would have a particularly detrimental effect - I just think it won't be of any interest to the manager interviewing you. Better to spend your time talking about things that show you are suitable for the job, instead.
Having an encyclopaedic knowledge of different TOCs, or different types of train, or the history of services/routes is actually not required in the slightest to be a driver, guard, signaller, or most other operational railway grades. The knowledge required to perform an operational role safely is very different from the knowledge possessed by a typical enthusiast/basher/etc.
I would suggest that just being a regular peak time commuter would probably be more help than being a full on enthusiast. Knowing why delays happen, how tickets work, how stations are laid out, etc would be far more useful than knowing all about irrelevant types of trains and sections of route you will never drive on.
If you are an enthusiast, and also possess the right skillset to work on the railway, then great! You will enjoy your work for sure, but just being an enthusiast does not mean anything. At the end of the day, there are countless drivers/guards/etc who had only travelled on a train a handful of times before they started on the railway, and they are excellent at their jobs. They receive the same training as everyone else.