I agree completely with what JohnnyOne says, as the driver recruitment team at Avanti almost certainly monitor these forums...
If you get there, my suggestion would be to use the materials provided by the OPC to prepare for the assessments and the CBI that comes at the end. I sat the MMI for my TOC, so I'm not 100% sure, but I think it's only 5 scenario based questions in the CBI. You'll need to answer those by drawing from your own experiences (work or personal), ideally using the STAR method, which is basically giving a comprehensive response that's packs in all the details without too much fluff. The interviewer may ask you ''and what was the outcome?'', or ''and how did that make you feel?'', and they might be trying to get a little more detail from you, or they may test your patience to a degree. The questions will focus on times where you've done/dealt with 'X', and the answers you give are used to determine your suitability for the role, through the examples themselves and how you put them across. Think about some of the things the job of a train driver entails and what they sometimes have to deal with, and how they do it.
Situation (what it was)
Task (what you had to do)
Action (what you did)
Result (what was the outcome)
Example questions have ended up on this forum before, and you could use those to get a rough idea of what to expect, but don't worry too much, get your head down, and focus on the practise/preparation materials.
