Depending on how late the Driver booked off on the previous shift, if there was no one 'spare' available to take it, I have no doubt it would be cancelled, as would the next working of that diagram.
There is also a problem that rolling stock may not end up in the ‘right’ place. The first working may result in the train going from A to B. Then the second working from B to C, where a different driver takes over. If the driver that is booked to move the train from A to B does not start their duty in time to move the train from A to B, there is no train at location C for that driver to drive. Unless other arrangements are made, this could affect many services throughout the day. Especially as if a train misses it’s booked timetabled path, either it or other trains will end up being delayed as in some places on some lines, the existing timetable is running the line/infrastructure at maximum capacity.
This is exactly the situation that the train company does not want to occur. Hence they would rather either completely cancel a service or run it fast/non-stop to try to maintain the rest of the timetabled workings.
On the subject of excessive working hours, railway companies have to demonstrate that they took reasonable measures and precautions by having a formal risk assessment done. Including appropriate documentation. If later on an incident occurred, the company could be prosecuted under H&S laws if it could be demonstrated that the company had either not properly carried out a risk assessment or that they had failed to comply with their risk assessment and internal procedures.
Hence most railway companies adopted the former British Railways ‘Hidden’ rules that BR introduced after the formal investigation by Anthony Hidden QC was published (“Investigation into the Clapham Junction Railway Accident” published by Her Majesty’s Stationery Office, ISBN 0 10 108202 9).
in his report, he makes a total of 93 recommendations. Of these recommendations, number 18 says “BR shall ensure that overtime is monitored so that no individual is working excessive levels of overtime.” And recommendation number 19 says “BR, in conjunction with the Unions, shall introduce the concept of scheduled hours within the Signals and Telecommunications Department in order to make better provision for work which has to be carried out at weekends.”
At the time, the S&T staff had a rostered working week of 39 or 40 hours not including Sundays (which were always overtime, as Sunday was not part of the working week according to the terms and conditions that had been previously agreed with the trade unions). However, it was common practice for many S&T staff to work overtime both beyond their normal hours and to work on their ‘rest’ days including Sundays. There were no limits at the time of the accident on how much overtime could be worked. As working Saturday and Sunday could double an employee’s take home pay, some staff would work all the overtime they could. Meaning no days off for months on end.
In paragraph 8.54 Hidden writes “the Investigation saw sample figures which showed that in the thirteen weeks preceding the accident 28% of the workforce worked seven days every week, another 34% worked thirteen days out of fourteen.”
Paragraph 8.55: “Mr Callander accepted in his evidence that continuous working of seven days a week every week was affecting the ability of the staff in their mental alertness.” “He thought it would be human nature that there would be some slippage of the high standards that the job required.”