Hi Jenny
Excellent news and fingers crossed you'll get through! A lot of good advice is to be found in the jobs and careers section generally, but I'd say:
1 Research the company on their website - get to know the key information such as number of employees, passengers and journeys, different routes, awards gained.
2 Buy a Bop-it Extreme and practise, practise, practise - this should help for your reaction test should you make it that far. (I understand there are computer games that can help but I'm not sure which - previous posts on the careers section will help)
3 Get hold of some psychometric testing books if you can - mechanics in particular. I looked at How2Become a Train Driver on Amazon and there were some useful suggestions under the "people who bought this book also bought..." section).
Best of luck and let me know how you get on!