There is going to be a big crunch on units in a few months time so they need some luck.... with the ramping up of testing, it would appear that more units are likely to be needed.
It would seem reasonable that the remainder of the fleet not being used for Tunnel testing or passenger service are simply parked at Old Oak Depot spare. I assume that as more units are upgraded to the next software type, there will be a return to operation for them.
With previous software updates they have learnt the hard way to have a fall back option, in the H&H shuttle case 7car or previous software with 9car in the short term but in the Heathrow case it would have to be the previous version of the software on 9 car units. Hence until they are sure there are no new problems they will probably keep a good number of the 9 car units with the previous software available.
That leads to 2 potential permutations
1. When they have the confidence in the new software (i.e. no need to fall back to 7car again) updating the unused units to the latest software and gradual introduction to Reading to replace the 7 car with 7car units then getting extended to 9 car then get used for core testing, next non core 9car software version testing or 9 car on the eastern side. (probably what they hope)
2. It doesn't get rolled out to reading quickly and some additional 9 car units get used for core testing which then make the logistics of the rest a bit harder which eventually make the logistics of lengthening the 7cars out east when the LST platform works are complete harder as OOC is full.
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