Hi Mullen,
My advice is to start with the basics. Turn up early, ironed shirt and tie, get a hair cut etc. Be presentable.
Look at the companies visions and values, this page should help:
https://www.scotrail.co.uk/about-scotrail
Have a look down the side tabs. The more you know and the more you can tell them limits the amount of questions they have to ask.
Know the job your going for, It's advantageous if you can shadow a member of staff doing that role, So get on the phone to the person interviewing you and ask to shadow for an hour or so giving your more opportunities to ask staff questions. This also shows them you're not mucking about.
If they ask you a question and you don't know the answer, Don't bull sh*t. They'll know you have no idea. They might ask you a few competency based questions like "Give me a time when..."
Mention that you want to progress through the company with your career. This shows them you're serious and you're not going to quit at the first chance.
At the end of the interview thank them for their time and if they say "Have you got any questions" Make sure you ask about career progression(even if you already know the answer, it reiterates to them you're serious), Ask when it is likely you will be contacted regarding your progress to the next stage.
Little extra info would be railcards on offer, basic ticket info (Cheap Day, Standard Day, Open Returns etc), What promotions ScotRail are running, Delay-Repay, Club 50.
Hope this helps.
Coming from a person who has had an interview for Customer Service and a Conductor Interview. Currently a Conductor for ScotRail.