In message , at 21:54:56 on
Wed, 16 Aug 2017, Recliner remarked:

I don't know if this was to reduce the number of
departures during the work. The departure board declared that
the train would be divided at Salisbury and that only three coaches
would go forward.

Again, not unusual.

Back in the day when I go that Eurostar connecting train from
Warminster, the regular Sprinters often used to split/join at
Salisbury (I went to Basingstoke a few times that routing).
--
Roland Perry