I don't think it's possible to turn off the horn on the cars that spawn on the roads. I've had the same problem when I stop a train too close to the crossing. If you hold them up, without movement, they'll honk as if to say, 'Hey, get the train outta here, so we can cross!' It only takes thirty seconds to a minute to trigger the anger in a driver at a railroad crossing in Trainz when you stop close enough to a crossing to trigger it and then don't move a muscle.