Firstly, well done for getting this far. It's not easy is it? I have recently been to the final assessment day and have been officered a position pending a medical. They told me that out of around 700 applicants, they've got down to around 70 doing the final day, so you've done great so far.
I can't give too much away as it wouldn't be fair on others, so I hope you understand.
On the day, you will do various things including the interview. Part of it is competency based questions, so the best advice I can give you is read the info that they have sent you, mainly the info about key skills they are looking for from a driver. It's on one of the attachments on the advert, 'JD Driver' I think it's called. Prepare for the interview by getting some examples of when you've dealt with the keys skills. Try to think about what they want from a driver and focus on those. Research competency interviews and it will give you tips on how to prepare for them. I found this useful:
How to handle competency-based interview questions
The other things you will do on the day are:
Climbing in and out of a tram from the ground. They show you the technique and you copy it.
Operating manual points. Basically pulling a large lever to change points in the depot. This is just so they can see you are able to do it as it takes a bit of effort to do. Nothing too bad though.
Group exercise - you will be split into teams and you will go through a scenario written down and work together. It's nothing to complicated, but I think it's to see how you work with others.
Tram simulator - This is where you will drive a virtual tram between 2 stops. This is to test your observation regarding speed limits and signals. Take your time and follow the instructions given. You will also have to do an announcement. It's really cool - a full size replica of a tram cab with all the buttons and switches. Make sure you follow instructions and look EVERYWHERE to make sure you don't miss anything.
I hope that helps and good luck. Just be yourself and enjoy. You've got this far, just one more step. Might see you soon!