I'm assuming it will either be related to rostering or train diagramming, mostly asking you knowledge of spreadsheets / excel, communication / interpersonal skills and how you would feel working with a system that has to operate within the strictest sets of rules, especially if it is traincrew rostering.
From my experience, Northern are very good at encouraging you with your answers and not putting you on the spot too much.... better than many companies. I wouldn't worry too much about knowledge of rosters or diagrams as most of the training will be provided by a mentor should you start.