If it's the latter then they are formed of the last two services of the previous day into Weymouth.
Which is fairly obvious else where would the stock come from.
One of which would have been a 158/7 as the 07.24d
TRO» was 4 car 158, I'm guessing that the additional 158 is attached at Westbury.
It's not necessarily obvious... I've seen (dare I say it!) dafter questions asked.
People ask questions because they would they like to know the answer. Obvious to one person is not necessarily obvious to everyone else.
Louis may have been genuinely asking where they came from; I don't know Louis so I don't know what level his knowledge of
FGW▸ land is.
The way I read the message I took it as having one of two possibly interpretations which is why I then asked him which he meant. I'm not sure your stinging responce was so justified.
To drag this back on topic; to confirm your guess Tramway, yes, the additional set is attached at Westbury to provide peak capacity on the corridor into Bath and Bristol.
On the other note, as per the workings show, the 2008 and 2308 arrivals at Weymouth stable for the evening, the 1913 arrival shunts into Jersey Sidings to alllow the 2008 arrival. The reason for the shunt is because the 1913 arrival and subsequent 2021 departure is formed of a 3-car 158 which is required back at
SPM▸ that evening. 158 starts its day from Fratton and works via Brighton, Malvern and Weymouth before finishing at Bristol.