Unless otherwise provided, the following signals must be displayed in the places provided to the rear of every train, as markers, to indicate the rear of the train:
By day, marker lamps lighted or unlighted, or green flags;
By night, marker lights lighted, displaying green to the front and side and red to the rear, except when train is clear of main track, green must be displayed to front side and rear.
One marker displayed will indicate the same as two but the display of two is required.
Exception: That part of this rule requiring markers to display green to rear when clear of main track does not apply in CTC territory.