Ian,
I have worked with slot cars in Aurora Model Motoring (originally) and Tyco (later, but after train set was apart; cars were easier on carpets!). I have always wanted to integrate Tyco US1 Trucking with trains, with interacting and intermixed live loads for both together (my car/train dream- a closed loop system!).
The one thing I am really intrigued with is the construction of a two or three parallel rail grade crossing- that would be something to think about! Have you considered more than one rail line on your grade crossing?
BTW, what slot car brand(s) are you working with?
Oh, and yes, commercial slot car grade crossings are all, to my knowledge, code 100. I think all the ones out there are old stock; I don't know if slot car manufacturer has made any for quite some years.