Discussions elsewhere a few months ago suggested that very few if any of the 456s will operate on the Windsor side. 8 or 9 will apparently be used on Guildford - Ascot shuttles, and most of the rest will work on the main suburban side (ie through Wimbledon), but not generally in 4+4+2 formations with 455s, as was expected.
SWT▸ insiders reckon a number will work paired up as half of an 8 car (4+2+2) formation, and that this then releases 455s to run additional peak trains, for example they plan to have a couple of extra services running in to Waterloo from Raynes Park.
The other point of course is that there's a continuing 455 overhaul programme for
DDA» mods, and a separate programme for retractioning, that also affect the number of units that are available for a two or three year period.
What I mean is that the 24 units (say 20 or 21 diagrammed daily) won't generally augment pairs of 455s up to 10 car, which was the popular assumption. SWT's announcement that the 456s were arriving listed only the following changes that can be considered 'suburban', most of the enhancements listed were to mainline longer distance services, presumably via cascaded 458s or 450s:
Suburban services:
0717 from Guildford via Cobham (8 car to 10 car)
0807 from Guildford via Cobham (8 car to 10 car)
0817 from Woking (8 car to 10 car)
1702 Guildford via Woking (8 car to 12 car)
1732 Guildford via Woking 8 car to 12 car)
1757 Twickenham via Kingston (8 car to 10 car)
1848 Guildford via Cobham (8 car to 10 car)
Windsor lines:
1705 Aldershot via Richmond (4 car to 8 car)
1713 Teddington via Richmond (8 car to 10 car)
Additional services:
0722 from Raynes Park
0922 from Raynes Park
AIUI▸ the 10 car Windsor side, (Windsor and Hounslow loop etc), is well within the capacity of the full 458/5 fleet, basically they are using 36 merged units to operate the routes that were previously covered by the 29 HC 450/5 conversions. Another relevant issue is that the 455/456 fleet cannot be used on the Windsor side 10 car routes where
SDO▸ is going to be used (ie minor stations beyond Staines towards Windsor) so their normal operating area has to be on routes where all platforms are extended - and that poses the difficulty that you have pointed out already that they cannot run into Waterloo P1-4. It seems the limited number of main suburban 10 car trains will use P5 and P6.
Paul