The question in my head is how long for?
The reason for that thinking is that 18 units could allow the release of 1/3 of the 444's at a time. In that if you did that there could be scope to do a semi major refit of them, potentially including extra seats by the removal of the micro buffet (which is rarely if ever used now). Three rows of seats (2+2) would gain you an extra 12 seats (which is broadly what would fit in that space, although it may be 4 rows).
That would give you a bit more capacity without compromising much.
It could be that there are other options that could further improve the layout of the 444's and by able to do them in sizable chunks could bring the full class back into service sooner.
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In a post in the SWML capacity discussion a few weeks back TEW found only 9 am peak trains not running at full length:
If those four 5.444s in the list were combined into two 10.444 pairs the two remaining trains could be 12.450, requires 6 x 450.
If the 8.450s in the list were extended to 12.450 there's another 5 x 450.
So I've found homes for 11 x 450 on am peak arrivals, and that's about it for strengthening in the morning peak. There are obviously more trains that arrive extremely early, and others that are still arriving towards 1000 that can also be lengthened.
I still cannot see there being space on the network for a random number of spare 442s, once the 707s are in service, planned to be completed within 2017.
There's also the 0643 Southampton to London 0839 (5-444) and 0753 Basingstoke to London 0853 (5-444). Although they would only use up another 2 units.
However it's not just trains heading into London in the morning peak that are busy there are trains heading away from London too.
As well as during the day there are still quite a few 4 or 5 coach services that could do with being longer, some of the fast services which call at Farnborough and services through Guildford. With more units these can also be strengthened without exceeding unit milages.
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Of course there's always the "option" of loco hauled 442's away from the third rail (how likely it is another matter)
Going back to the slightly crazy suggestion of loco hauled 442's, a return run to Exeter would require 8 units (with a further 4 to run it doubled up to Salisbury).
Then a further 4 to Salisbury (+2 for doubled up to Basingstoke).
Which at 18 would leave no spare units but every single service run full length all day between Basingstoke and London and every Exeter service is full length between Salisbury and London, so is probably some score to run shorter services during the day to enable there be some spare units.
Alternatively you retain a few 158/159's to run services where they extend beyond the above (I.e. those to Bristol and Yeovil Pen Mill/Westbury) and extend the double running of the 442's further (say Yeovil rather than Salisbury and Salisbury rather than Basingstoke) and/or extend the services further (I.e Yeovil rather than Salisbury and on an equally crazy level Okehampton rather Exeter).