Are trucks required to stop at railroad crossings?

At a level crossing, the same rules apply to trucks as do to cars, in that the Train always has the right of way. As long as there's no indication of a train coming, (lights flashing, etc) the truck does not have to stop.

However, some trucks, particularly those carrying dangerous goods, are required to stop regardless of whether or not the lights are flashing, to check to make sure there is in fact no train coming. If this is the case, there will be a sign on the back of the truck warning that it stops at all railroad crossings.