The idea is to see if a candidate is suitable for a role by seeing how they react to situations, ability to think on their feet and to use real life situations they've dealt with in previous jobs etc.
A lot of railway jobs involve dealing with situations which aren't straight forward every day.
My opinion on all online situation tests, interview questions and OPC tests is that you shouldn't need to revise, know the questions in advance and so on.
The idea is to see if you're suitable for a role, not to see who's capable of giving the right answers because theyve learnt what to say/do to pass the phases of the recruitment process. I'm told you often find a lot of people who do just this then get offered a role and struggle.
Hope this helps!