Every motor vehicle, trailer, semitrailer and pole trailer and any other vehicle which is being drawn at the end of a train of vehicles shall be equipped with at least one tail lamp mounted on the rear which, when lighted as herein required, shall emit a red light plainly visible from a distance of five hundred feet to the rear; provided, that in the case of a train of vehicles only the tail lamp on the rearmost vehicle need actually be seen from the distance specified.