DMI's are a tricky one on here, its quite personal - questions you get asked are based on the personality questionnaire you answer before the DMI, so people can't really help you with them, though they are loosely based on questions similar to the MMI.
Other than that, research MTR Crossrail, they wanna know everything you know about, well (MTR) Crossrail... If you take a moment to search MTR DMI on here, there is some valuable info which no-one seems to ever find.
The DMI is down to you, people may be reluctant to help because they personally put so much hard work in for themselves, and the only way to really help you is to give you everything they've personally spent hours working hard to find themselves (if that makes sense?) You have to study, and research, then remember it all. Google is your best friend. Everything you find 'MTR Crossrail' is important, nothing is too small and insignificant, nothing too far away and worldly!
Good news, it was the most fun interview I've ever experienced in my life, I too am rubbish at interviews, but they make you feel so at ease. It's more like a 'chat' than a formal interview, but of course don't be late and wear smart clothes.