Question why do SWT▸ run a 159 from Long Valley sidngs into platform 1 to form the 17:24 all stations Southampton? Is it the Lymington unit changing over?
Simple explanation, it's just a peak extra, one of a handful that leave Basingstoke in a few xx24 slots in the late afternoon, to augment the service towards Southampton or Portsmouth. Nothing to do with Lymington, that sees one 158 allocated for the 5 day period, running down on Monday and back to Salisbury on Friday night.
Complicated explanation.
The normal off-peak fast service on the route is xx05 and xx35 to Weymouth, with semi fasts at xx09 to Portsmouth via Eastleigh, and xx39 to Poole. However, in the afternoon peak period for about three hours everything changes; the xx05 and xx35 run as 10 car but now split at Southampton (or Bournemouth) into fast and slow portions, the latter following behind the former at minimum headway. This provides 4 tph west of Southampton, of which two are fast, and two are semi fast to Poole, up from the off-peak 3 train service.
The normal xx09 and xx39 semi fasts from Waterloo now provide a half hourly Portsmouth via Eastleigh service for about 3 hours, up from the normal hourly service.
With me so far? What it does is use 4 departures from Waterloo to provide 6 services further west, IYSWIM.
The result of the above is therefore a gap in the normal semi fast pattern between Eastleigh and Southampton, and this
DMU▸ fills one of those gaps. There are a few other DMU 'shorts' about that also fill gaps here and there, or double up an otherwise hourly service in the peak flow direction, but in an ideal world they should probably all be
EMUs▸ , and are expected to be once the 458/5 conversions are finished. In the franchise bumph these extras are collectively the 'South Hampshire Locals'.
There's one caveat though, there's presently a Winchester to Totton stopper that is a 159 because it has to use the non electrified Totton sidings.
The usual follow up to this subject is that people question just how many spare DMUs SWT might have (for use in Wiltshire perhaps) if they didn't run all these short workings around the
DC▸ area. Ignoring the Lymington shuttle,
AFAICT▸ these are all 158 or 159:
2L88 0642 Basingstoke to Waterloo 6.159
2B89 0655 Basingstoke to Southampton Ctrl 3.159
2B93 1724 Basingstoke to Southampton Ctrl 3.159
2T69 2221 Basingstoke to Portsmouth Hbr 3.159
2T75 0030 Eastleigh to Portsmouth Hbr 2.158
2T14 0500 Portsmouth Hbr to Basingstoke 6.159
2E24 0933 Portsmouth Hbr to Southampton Ctrl 3.159
2T92 1649 Portsmouth Hbr to Eastleigh 2.158
2B98 0542 Southampton Ctrl to Eastleigh 6.159
2E13 0751 Southampton Ctrl to Portsmouth Hbr 3.159
2B94 1817 Southampton Ctrl to Winchester 3.159
2B91 1638 Winchester to Totton 3.159
2B95 1905 Winchester to Southampton Ctrl 3.159
Hope this is of (slight) interest...
Paul