This is a common mistake people make. Just being in a safety critical role doesn’t give you any additional points, if anything it gives them more room to scrutinise you. Your best bet is to demonstrate that you’re able to meet their needs out of a railway/safety critical environment. It’s about demonstrating that you’re competent as a person rather than you being able to act competent at work. I work in a safety critical role and didn’t mention it once. I used other examples from different walks of life and passed. Loads of people who work in safety critical work fail the CBI/MMI because they rely on it too much. Your best bet is to demonstrate that you can be competent in any area of your life and apply that to the question. Also be mindful that they Will likely have more knowledge pertaining to the railway so if you make even a single mistake in your explanation, it will stick out like a sore thumb whereas if you describe an unfamiliar field to them, it leaves more room for error.
That’s just my opinion though and what I’ve deduced from loads of conductors failing the interview stage. Speak about whatever you feel confident and happy to talk about, maybe even something that engoies genuine emotion from you like happiness, annoyance, anything.
Use varied examples from wildly different things as well if you can. Most importantly, be honest. Even when they ask “and how did that turn out”, you don’t always have to give a grand answer. Even if it didn’t turn out as well as it could have gone, it’s still a learning experience. Good luck to everyone else who still has their CBI to do and to all people working in the railway or safety critical roles, don’t think that’s an automatic shoe in because it’s not, it’s about competency not a CV interview.