I'm guessing you've gone for the OBS Eastbourne position? The online test will include about a 40 minutes maths test with varying prices of tickets with discounts and you have to work these out quickly and efficiently. I can't remember how this test begins to be fair so sorry I can't help that much.
From what I remember on the assessment days, it begins with a group discussion with you and your team setting up a fictitious health and safety briefing. You'll be given a list each and firstly on your own you will choose your top 6 choices. After 5 minutes or so you and the team will discuss each others choices and as a group deciding on the top 6 with one person feeding this back to the assessors. What they're looking for on here is the candidates listening skills to the other team, those who are able to get their point across in a dignified manner and be willing to change your views if you feel the other person has a good point. Don't sit in silence!
The next stage is the trainability test where you're given 8 paragraphs to read and remember. Make sure you read these through carefully but also quickly. These will be taken from you and you'll sit there for 5 minutes then the test paper will be given out. after a pause you'll have to answer questions based on the paragraphs you've just read. These are multiple choice. These questions will have subtle differences. For example: what colour was the fire exit sign?
A- white background with green 'exit'
B- white background with green 'EXIT'
C- green background with white ''EXIT'
D- green background with white 'exit'
Providing you pass this stage you'll then go into role play.
You're given time to write down a vague plan of how you'll approach this as you'll be given material to work from. This will be taken from you whilst you wait your turn. When it's your turn you will be given your plan back to you and then a scenario will also be given to you which changes the aspect of things such as trains being delayed by an hour etc.
You'll go back into the room where a mock train carriage awaits you. Walk in make your announcement then patrol the trains. They're looking for good posture, how you speak, how you deal with the confused or agitated 'customer' (the assessor), if you make a mistake they want to see how quickly you recover yourself. It's less daunting than it sounds.
Good luck with it all