If the crossing is done as kind mocrossing they should automatically stop. The code is built into Trainz. Triggers at about 80 meters or so I believe. Ok - once in a while a car or two won't (kids running the gates I guess, lol).
The most obvious reason they don't stop is that you may not have installed the crossing correctly. You must attach both the track splines and the road splines to the spline points on the level crossing object.
