1979 ? A few years after the last of the Westerns and Hymeks, alas. Class 50 did most of the WofE expresses, Class 47 the rest, but this was the time they were getting replaced by HSTs, which took some years to change over.
There traditionally had been very little freight on the Reading-Taunton line, but as the large quarries developed around Frome then Westbury became a centre, especially for block trains to London. A good number of Class 56 would have been allocated to this work at the time.
Bristol to Portsmouth trains would have been Class 33 by then, some with Mk 1 stock, others with spare 4TC units. I can't recall if Hampshire demus were still getting through to Bristol at the time, but I think not. Services only as far as Salisbury, as well as Weymouth trains, would be Western dmu. Crews were commonly changed at Westbury, which had long been a significant WR depot. The dmus would be a mixture of the Gloucester cross-country units (119), the Swindon units (120), and the Derby suburban units (116) from Bristol St Philips depot. Unfortunately Bristol had always had a habit of allocating sets somewhat indiscriminately, a suburban unit would be on Weymouth turns while a cross country unit would spend the day on Severn Beach. This was true also of the Taunton to Bristol, and Minehead branch, services. There were also some Pressed Steel single units used in multiple. The Maiden Newton-Bridport branch used these, and the crews for this came from Westbury depot and thus spent nearly half their shift travelling on the cushions to and fro.
Some Weymouth services in summer had always changed over to loco hauled, mainly Class 31 by this time; they used to be Hymeks. In the winter they were all dmu.
The original line through Westbury was the Wilts Somerset & Weymouth, from Chippenham to Weymouth. It wasn't until 1906 that the line from Reading was opened. This joined just east of the station by a tight curve, and the non-stop expresses, especially on summer Saturdays, were quite a nuisance, so in the 1930s the Westbury Avoider was built. This is the line that passes south of the station, and over (not under) the road. It's quite a lengthy stretch of line, several miles between the end junctions. It's completion allowed the Cornish Riviera to be speeded up, nonstop from Paddington to Plymouth, from 4 hours 7 minutes to the even 4 hours. I've never actually noticed that trains on it were visible from the station. Indicative of how far out of town the station is, it passes between the station and the built up area. For a long time most of the expresses to the West were nonstop between Reading and Taunton, it's only since the 1960s that there has been more than odd stops at places like Newbury, Westbury, or (most notably) Castle Cary.