Perhaps I’m not explaining myself.
GTR, like many other operators, have historically struggled for crew in the run up to Christmas, and particularly for Saturdays and late turns towards the end of the week. As anyone who is involved in traincrew resourcing know, this is because it becomes more difficult to fill gaps in the roster as crew are more reluctant to do rest days and overtime - understandably they have family commitments etc.
For the last few s GTR has been particular affected because they have had a higher than normal level of training to do over (several new fleets, new routes, ETCS, ATO, rebuilt London Bridge, Kings Cross, etc), and has thus been stretched further for resource. Add in the industrial relations issues a few years back and that was another factor.
This year, all those issues would largely have been over. Even with Covid, the training has been going reasonably well. However according to someone I know, GTR traincrew have been affected by Covid more than almost any other operator (on a %age basis), and have been running tight for months. The timetable change last weekend helped to address this, and freed up resource for the anticipated usual level of ‘pre Christmas difficulty’. However Omicron has hit them hard (London being well ahead of the curve) and thus numbers off are quite astonishing.