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Some of the not in passenger service units are used for training. Based on reports on this thread, 002, 015, 024 and 028 seem most likely to be in this category. Excluding those makes the not in passenger service split between 001-030 and 031-060 roughly even.
If we ignore the training units and those still stored then the difference is less dramatic, but still a difference.
701001 - 701030
In service 16 so just over 50%
Training 701002, 701015, 701024 & 701028 (total 4)
Stored 701003 (Derby), 701014 (Eastleigh), 701016 (Eastleigh) & 701027 (Eastlaigh) (total 4)
Being prepared 701003 (Derby), 701004, 701008, 701019, 701023, 701024 & 701029 (
total 6)
701031 - 707060
In service 26 so not far off 100%
Training none
Stored none
Being prepared 701032, 701040, 701045 & 701055 (
total 4)
Half as many again still being prepared for service in the first half of the 701/0 fleet compared to the second half which is reasonably even.
With a further eight 701/0s in the first half also not in service compared to none in the second half is the significant difference.
Note I count 701028 as training as it only had a very brief stint in service but can at least be argued t be "alive".
Now if I knew which ones were being used for any tests some of the differences might be nearer to even.