If you are referring to Deadhead time…
Following the above excellent explanation of times we are allowed to work, lets say my crew end up leaving the yard and working an industry.
We reach the 12 hour limit with no time to return the locomotive to the yard.(this applies to road crews also)
Of course, we can all tell time and have watches so we have made plans to have the locomotive or train in a siding before we reach the maximum work time allowed, and we have called a cab or yard limo to come pick us up.
Because we are still “on duty” we are still being paid, but due to the hours of service law, we can perform no service whatsoever to or for the carrier after the 12 hour limit.
We must note for the FRA the time we reached the 12 hour limit, when the cab arrived to pick us up, when we reached our point of tie up, either the away terminal or home terminal, and when we exited the cab or yard limo.
At that point, we are no longer on duty and pay ceases, and our “rest time” begins.
We are not even allowed to tie up in the computer, we must do this function the next time we begin a duty cycle.
If you are on a crew that always reaches the hours of service, then this becomes the norm for you, in that you adapt to the constant change in when you get up and when you go to sleep.
If you are on a yard crew or an assigned job with regular work hours, and “the hogs getcha” infrequently, then it can cause minor problems for a day or two.