Why Northern, why you want to be a driver, what you know about the role, what do you know about the company. Some info about your local station is always useful in case they ask.
I prepped STAR based answers to the following questions (which was useful for MMI too):
1 - Describe a time where you've had to follow rules and procedures
2 - Describe a time where you've been involved in an emergency / unexpected situation
3 - Describe a time you've been on a training course for over 2 weeks
4 - Describe your greatest achievement
5 - Describe a time where you've had to work for long periods of time alone
6 - Describe a time you've had to solve a difficult problem
7 - Give an example of a time you undertook a task similar to train driver's role
8 -Describe a time when you have worked to a very high standard whilst encountering difficulty
9- Give an example where you've had to work under pressure.
10 - Give an example where you've had to communicate an important message
11- Give an example where you've had to work as part of a team (to achieve a task)
12 - Give an example of when you provided high level of customer care and service
13 - Give an example of dealing with difficult and aggressive people
14 - Give an example of working with people from different backgrounds
15 - Give an example of a project you had to complete and the obstacles you had to overcome
16 - Describe a time when you had a disagreement with your manager and how it was resolved
They obviously might not be exactly worded that way, and if you are struggling for an example, you can always ask if there’s another question (they offered me the choice of another question, and I asked what questions were left so I could best use my examples to the appropriate questions).
Also useful to prep a few (sensible!) questions to ask your interviewers too.
Good luck!