Of about 27 seats per unit, which when run as a 9 coach train could be a drop of 81 seats.
It could be possible to have 185's to be used by some of the off peak services so as to release more 159's to be used during the peaks.
The only reason you may do it is as a unit swap so that SWT ends up with more 2 coach 158's in exchange for some of their 159's to enable more peak hour services to run as 10 coach trains. In doing so there would then be a need for extra units to cover for some of the capacity loss from the "shortening" of the SWT 15x fleet, this could then be covered by the 185's.
It would also allow services, such as the Salisbury 6 (Romsey, Southampton, Salisbury), to be run by 185's freeing up 158's to be used for London services and/or move elsewhere.
However, if Exeter services are run by 10 coach trains with 6 coaches (a pair of 159's) going beyond Salisbury and 4 coaches (a pair of 158's) turning around to form the next train back (or the service to Bristol). There wouldn't be the need for that many extra 158's, as most of the 158's would be limited to running between Waterloo and Salisbury.
Then the rest of the services could be run as 10 coach 158's/159's or 9 coach 185's (timetabled to be on services which are not quite as busy) with some 185's being used to run the Salisbury 6 services (possibly on an increased frequency of 2tph, which may only need 5 units rather than the current 3 if turnarounds are made a bit tighter).
Although that may be until circa 2025 (plus or minus a few years) when the following franchise could start to use EMU's (depending on electrification) for services (at least east and south of Salisbury).
Maintenance shouldn't be too much of a problem with SWT as the 185's should share quite a bit of kit as the 444's and 450's so shouldn't be too much extra work to maintain them.
However it could cause some problems with confusion between 158's and 185's if people aren't being careful with what they write!
Although the current SWR haven't taken on the 185's (given the fact that there not going to be available for a bit that makes sense) I still think that it could be a good place for them (as suggested above) at it frees up Sprinters to be used elsewhere without (as far as I can tell) needing to travel slower due to higher sprinter speed limits. It would also be in a franchise where a bit of a higher fuel bill wouldn't make a lot of difference.
In fact other than the restriction in capacity (the are definitely some diagrams where that wouldn't matter as they are unlike to be full) and the fact that you'd have two fleets of DMU's that may not be able to work together (although that could be fixed if needed) I'm not sure if any significant problems with then moving there.
However on the plus side you'd have:
- more 10 coach services to/from Waterloo
- 185 operation of the Salisbury 6 services (Salisbury to Romsey via Eastleigh)
- scope for longer trains to Exeter
- scope for longer/more frequent Bristol to Waterloo services
- score for services to Weymouth via Yeovil as an alternative (to Bristol) use of units on the Exeter services (although not faster that the current services not so slow that you'd get to Weymouth sooner by waiting for the next existing service if more than about 10 minutes later as due to the different calling patterns could be a big advantage for those traveling to/from Clapham Junction, Woking and Basingstoke)
- releases 159's for use elsewhere (although probably only half as many as the number of 185's that head there) which given that they are effectively 3 coach 158's would make them very useful to a lot of franchises. Especially if they allowed the 158's that were formed up of 1.5 units to be split back to being 2 coach units again.
On that final point you could probably release 12*159's to go to GWR to allow their 12*158/9's (3 coaches) to be converted back to 18 units with 2 coaches. It would probably require two diagrams normally run by 159's to be replaced by 185's, the Salisbury 6 services to be converted to 185 operation and all the 158's paired up to replace a single 159 to form 7 or 10 coach services.
It would be tight, especially if extra services (more Bristol or a new service to Weymouth via Yeovil) were introduced, but probably could be done.
[WISPER TO AVOID HAND BEING BITTEN OFF] 18 class 158's anyone? [/WISPER TO AVOID HAND BEING BITTEN OFF]