I think if people were to give you specific questions they had then personally I think it would be counterproductive as you would focus on that when the reality is your questions are likely to be worded differently!
What I would say is have plenty of examples of when you’ve worked alone, as part of a team, dealt with a difficult situation, been asked to carry out a task you did not think was right, followed instruction, asked others to follow instruction, a time when you’ve been able to help someone, a time when you have wanted to help but haven’t been able to, how you have maintained concentration for extended periods etc etc etc. then when the question is asked you can use the right one to answer.
Those are the types of questions (obviously worded in a much more subtle way) you will get. Be sure to answer using the STAR technique.
My assumption is your external so although you don’t need to be an expert on all things GWR it would help you stand out if you at least had a working knowledge of what traction and stations your potential depot serves.
Look at GWRs mission statement and any other company related information regarding the way the business is looking to go there is plenty in the public domain and use it to your advantage.