It used to be a lot different pre-covid and pre-SWR when the diesel fleet were not used as intensively. On a Friday multiple trains from Exeter up to London had more coaches attached at Salisbury than a Monday-Thursday. For example, you'd get a 158+159 attached instead of just the 159.Regarding SWR West of England services on a Friday all the stock formations in/out of Waterloo are exactly the same the only difference at the London end is the 1720 Front 5 runs on to Exeter instead of Front 3 and the 2340 runs on to Yevoil Jun.