RULE 31. ZONE RULE.     Conductors and brakemen in pool or irregular freight service may be called to make short trips and turn-arounds with the understanding that one or more turn-around trips may be started out of the same terminal and paid actual miles with a minimum of 100 miles for a day, provided (1) that the mileage of all the trips does not exceed 100 miles, (2) that the distance run from the terminal to the turning point does not exceed 25 miles, and (3) that conductors and brakemen shall not be required to begin work on a succeeding trip out of the initial terminal after having been on duty 8 consecutive hours, except as a new day subject to the first in first out rule or practice.