Hello mate!
I'm a driver at NXG. All I would suggest for the DMI is just be relaxed and confident, but not cocky. It's the first opportunity for ARL to get to know you as a person, not just a number going through assessments.
Research the company, the structure, how TFL and ARL work together, new routes and types of train coming in the future, which linesgo where etc.
It's a time for you to show who you are! Ask questions, try and think of at least two before you go in there. Show that you're willing to learn and are excited about the opportunity.