Is this an assesment day, or just a straight interview?
If it involves the paper tests I presume it's the usual 'weeding out' day, which should entail several sets of tests (like your practise papers but with significantly more detail) followed by some type of 'group activity', which, thankfully or otherwise, isn't an orgy like it sounds, but actually a way of testing your communication/teamworking skills and suchlike. This can apparently take one of several different forms, such as a group discussion and so on. If they still like you after all that, then you should finally get to do an interview!
Be aware that XC are notorious for being 'outside the box' in their methods (many on here will tell you that it isn't particularly fair, and is based on the 'professional' opinions of people who don't do the job themselves :roll

and it won't just be your performance on paper that they are interested in. Don't let that dampen your spirit, best advice would be stay calm and concentrate for the written stuff, and participate in everything else. Be relaxed and cheerful, but don't dominate, and if you have an interest in railways that's fab but keep it to yourself! Nothing they hate more than mildly stuck up 'spotter' types! (I'm not suggesting that you are one, just making a worthwhile point!

)
Good luck!
