Usual structure is rendezvous at about 0900, go through introductions, fire procedures etc, and an overview of the day. You sit two tests, the Ticket Checking and Selling Test and the Rules Aquisition Aptitude Test, then take a short break. A couple more candidates might then join at 1000ish for a Group Assessment where you have to sit down as a group and discuss a given scenario while the assessors mark you against a set of criteria.
Once all the assessments are done they're marked and they'll send home any who haven't passed them, and begin interviewing those who have.
Which depot have you got an assessment for?