Sometimes freight planned to use the avoiding line travels via the station, and vice versa. There have been problems recently with the up line points at Heywood Road Jn, so additional freight (and up GW IETs) have been travelling via the station.
Best to look on Real Time Trains to plan your day. A surprising amount of freight travls up through Trowbridge; not everything from Whatley/Merehead goes via the Berks & Hants. Also a number of freights reverse on the down reception line alongside the station, if travelling from the west (or the quarries) towards Eastleigh/Southampton.
Was there for about 30 mins this morning and saw a couple of freights.