Clearances That May Be Decreased. The overhead and side clearances prescribed in these regulations may be decreased to the extent of a line extending diagonally downward from a point 4 feet from the center line of track 22 feet above the top of the rail to a point 8 feet 6 inches from the center line of track at 15 feet 3 inches above the top of the rail. For tracks located at entrances to and inside buildings with 18 feet overhead and 8 feet side clearance as prescribed in these regulations, the overhead and side clearances may be decreased to the extent of a line extending diagonally downward from a point 4 feet from the center line of track 18 feet above the top of the rail to a point 8 feet from the center line of track at 14 feet above the top of the rail. Canopies at one side of track at freight platforms may be constructed at not less than 4 feet from the center line of track provided: The height of it is at least 17 feet 6 inches above the top of the rail; and Either of the following apply: A full clearance of 8 feet 6 inches is maintained on the opposite side of the track from the canopy, or Track centers to adjacent track are not less than 13 feet 6 inches.