RULE 69, WHEN NOT HELD FOR REST. (a) When engineers have a sufficient number of hours to work before being tied up under the law to run over the district on which the trip is to be made at a speed of 15 miles per hour, except 6 hours on the return portion of the double, Cheyenne to Laramie to Cheyenne, they will not be held for rest.

(b) When no crews are available under the law or under the provisions of Section (a) of this rule and the Company elects to use crews not having full off duty period of rest, no crew will lose its turn, which shall have sufficient time to cover the district over which the trip is to be made at a speed of 20 miles per hour, computed on constructive mileage when constructive mileage applies.