Alot of the interview will be example based, give us an example of when you've given good customer service or solved a difficult situation.
while I've no experience of Northern Rail, I'd say be ready for this.
May be worth a bit of web searching on "competency based interviews" which a lot of transport organisations now use.
i.e. rather than the more traditional "what would you do if X happened" sort of question, the questions are often now "give an example of a time you did X"
Re-read the stuff in the job advert / description / person spec (or whatever you saw before you applied) and if they have listed skills / experience they are looking for, then think in advance about a good example you can use for each one - trying to think of one during the interview is less good.
If it's a customer facing role, then things like dealing with difficult / challenging customers, providing good customer service, dealing with multiple and conflicting demands are likely to come up. As (depending on the role) may following rules / procedures, reacting to emergencies, something safety related - e.g. dealing with a safety related incident / potential incident. And either team working, or working on your own (or maybe both.)
May be worth researching the 'STAR' concept here - Situation, Task, Action, Result (i.e. quick description of the overall Situation? What was the Task that YOU did? What Action did YOU take? What was the Result?) - and they may also ask what you learned from the experience, and / or what you would do differently another time.
Bear in mind that the 'situation' bit is about the organisation / team, but for the task / action bits this is largely about you as an individual.
At its best, it's little more than talking about your past work experience. (and don't forget that if you have gained any of the relevant skills either through study, voluntary work or caring responsibilities, these can be just as valid as examples in your past employment.)
Some interviewers are more flexible than others here - some will happily accept a couple of examples, some will accept an answer based on "I've not done something that exactly fits your description, but I have a lot of experience doing something very similar" (and then describe that.)
At its worst can consist of very narrow questions asked by someone who knows nothing about the job, and if you don't use all the right buzzwords (right in this concept being the ones they have written down already) then you are likely not to pass.
Hope all goes well.
