Can't speak with anything like recent experience of Network Rail, and others may have more specific suggestions, but competency interviewing is usually questions along the line of 'give me an example of a time you did X'
The 'X' situations are usually things that they have mentioned in the job advert / person specification - can be a specific technical skill, or for a customer service role, it's more likely to be something like 'a time you dealt with a difficult customer' or 'a time you dealt with an emergency' or 'a time you delivered excellent customer service' and the like.
Experience of assisting people with disabilities (which means more than just wheelchair users) may come in to it.
Generally, one good example is better than a lot, or a long pause while you try to think of the best one, so probably worth thinking about examples you can use, but may be inadvisable to rehearse too much, in case there's a different slant to the question to what you're expecting. Sometimes an interviewer will add something like 'and what did you learn from the experience?' or 'and what would you do differently another time?'
There is likely to be some sort of safety question - may be about a time you have worked in an environment with safety rules, or a time you handled or prevented an incident or near miss.
Depending on your background, there can be times when examples from outside paid work (e.g. volunteering, study, caring responsibilities) may be the best answer - they don't always have to be from past jobs.
I've had mixed experiences of competency interviews - sometimes it's felt more like a chat down the pub (although without liquid refreshment) about what you've done in past jobs, and what they do. sometimes it's been someone from HR who knows very little about the job, asking very narrow questions and basically scoring you on whether you come out with the right buzz words that are on their list.
Hope all goes well.
