On the UP they used to start the numbers over again at midnight Omaha time for the various authorities (track and time, foul time, track permit, track warrant, etc) as well as with track warrants for track bulletins (which convey no authority, but you better have them!). The very first one issued after midnight would be number 1, the next one issued would be number 2, and so on. It seems like they would get in the upper 1000s by the end of the day before resetting at midnight.

Some months back that changed, as apparently one might be able to guess the authority number being issued based on one previously issued, even if one didn't actually hear the authority number. They have now gone to having a random number generator pick out the number every time one of these authorities is issued. Not sure if some incident prompted this change, but they are a knee jerk reaction type of an outfit...