Hi Tracktraveler,
For the interview do your research on Northern - company stuff like knowing who David Brown is, number of services, number of stations operated, and other franchise stuff like new trains & northern connect; a little bit of knowledge about the depot you have applied for eg number of links, how many conductors, routes and traction signed; and stuff on the role itself - the main priorities being 1) safety 2) performance 3) revenue.
Also prep some examples of things that have happened/ you have done from previous roles - around things like "tell us a time of when you were under pressure" or had to deal with a difficult customer/ person. Remember STAR - situation, task, actions, response.
And don't forget the "why do you want this job" question - be positive eg personal development, career change etc
For the group session, the group will be given a discussion point eg "Come up with a way to improve customer service at Northern", and you get about 15 minutes to discuss as a group, before 1 (or more) people feed back the group decision to the assessors. For this section the assessors are looking for participation and people who will listen to other viewpoints.
Good luck!