From friends it isn't just the railway that work like this. I have two friends that work in local government where their contract states they will work a certain number of hours a month, since they are both "on call", so they could work 1 hours in three weeks, but then can be "not available" after working 20 hours in week four. Anything over 140 hours is paid as overtime if they wish to work it.
Agreed, and I did it once due to my relief being snowbound, and it needed both my LOM and the District Manager to sign it off.
(The line still shut though!)
And signallers on a 12 week, 12 hour roster get a extra day off every 12 weeks so their average works out at 35 hours a week,