In this chapter, the following terms have the meanings indicated. Terms Defined. “Height of a freight car” means the distance between the top of the rail and the top of the running board. “Main track” means track on which the authorized speed of freight trains is in excess of 30 miles per hour. “Overhead clearance” means the vertical distance from the level of the top of the highest rail to a structure or obstruction above. “Side of a freight car” means that part or appurtenance of a car at the maximum distance measured at right angles from the center line of the car. “Side clearance” means the shortest distance from the center line of track to a structure or obstruction at the side of track. “Width of a freight car” means twice the distance from the center line to the side of a car as defined in these regulations.