The above info is all very well, but on
Saturdays as per the original request the current diagrams (as from May 2010) only have three 158s in use, the three used on the Salisbury to Romseys. Same on Sundays. So there seems little point in a special visit to see them at the weekend.
During the week you'd need to get around a bit, although Salisbury would see most of the 8 in use at some stage there's a couple which don't get there, the Lymington shuttle and another that runs from and back to Fratton on services within South Hants.
However there are now NO 158s normally diagrammed on SWT services beyond Salisbury, either to Bristol or Exeter.
The above doesn't include the FGW daily loan unit, I haven't any current info about that one.
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10 car surely - 12x23m won't fit the platforms (some needed lengthening for 10car formations
Correct, and even then there is only one 10 car service to and from Waterloo in the morning peak.