Why should I stop at a crossing?
Given a train´s weight and natural tendency to continue in movement, stopping at intersections to grant priority to smaller vehicles becomes a highly complicated task, hence, at level-crossings, trains always have priority over smaller vehicles.
Additionally, a train will always carry more freight than any kind of vehicle utilized for cargo, so the train´s priority over smaller vehicles is most suitable even from a social standpoint.
As evidenced, both the laws of physics and transit laws are always on the side of the train.