RULE 42. WHEN NOT HELD FOR REST--POOL FREIGHT SERVICE. (a) Crews with less than full rest period will be used in their turn if they have time to make the trip over the district at a speed of 15 miles per hour.  

     (b)  If the company elects to use crews with less time than provided in Section (a) of this rule, no crew will lose its turn out if the trip can be made at a speed of 20 miles per hour.  

     (c)  If the company elects to use crews with less time than herein provided, it will be permissible to use crews out of turn when necessary to meet the requirements of the service.