Hi there. As far as I am aware it should always be the assessment centre before your interview. I didn't get asked any questions about scotrail in my interview but I know another candidate who was asked a lot about scotrail. It's very hard to predict. As for the process what you have done up to now sounds about right, do you have a date for your next appointment? If you are in any doubt ring recruitment, they are very helpful and extremely friendly. Good luck!