RULE 57. LAP BACK AND SIDE TRIPS.     Crews on regularly assigned local or mixed runs of less than 100 miles required to make side or lap back trips not included in their assignment will be compensated as follows:  

     (a) Actual mileage of side or lap back trips in addition to minimum day for service on assignment.  

     (b) If total actual miles run is less than 100 miles, overtime will begin at the expiration of 8 hours; if over 100 miles are run, overtime will be computed on basis of miles divided by 12 1/2.  

     (c) When overtime accrues allowance will be made on overtime basis or as provided in paragraph (a) above, whichever is the greater.