The interview process will almost certainly, follow the STAR process - Situation, Task, Action, Result and you response will be scored against others.
Your answer does not have to be work related, and I would maybe say include situations that did not go to plan, but what you have learnt from them.
I am sure safety will also be covered so do some homework on the Network Rail Life Saving Rules - nothing too detailed but just an awareness of what they are and how they are applicable. Network Rail use Ellipse to management asset data, so maybe google it or talk about how you managed asset data in your current role and how important it is. Perhaps look into how you organise safe access in your current role, use of buried services, dealing with 3rd parties to organise safe access (I would think you use road closures on fibre planning). Maybe do a bit of high level research into the signalling assets, their types etc. nothing too nerdy though. Possibly look at PTS rules so you have a bit of a understanding of track side safety.
Read the JD also in case they ask you some questions what you know about it.
Interviews are never a good place for anyone, and you will forget 60% of the stuff you have prepared for - but most people will be in the same boat. What Route is the role in?