OK, so it's maintenance based I assume? I imagine they'll shoot off a few standard mechanical questions with a little bit of basic principles - so you should brush up on things like basic statics and dynamics theory. They might show you a simple mechanism and ask you to draw a free-body diagram. It might be something as simple as showing you a photo or drawing of an assembly or system and ask you what you think the parts are, where it goes and what the components do or how they're loaded.
It's unlikely they'll ask you anything heavy with mathematics or engineering theory as it's a trainee job, so they'll assume that you can learn on the job.
As for rail specific questions - they might ask you if you can define some basic railway terms and principles. It all depends on the level of candidate they're looking for.