FACT - a train can be turned around in 12 minutes.
FICTION - it takes 40 minutes to turn around a train.

YES - it IS possible to turn around the TransWilts train at Chippenham without interferring with the main line train.

Yesterday evening - the 17:02 from Melksham was to be terminated at Melksham.

It followed the 17:25 to Paddington into the London bound platform, arriving in there at 17:30. It left again (back towards Westbury) at 17:33. The signals for a Bath train from the other platform cleared at 17:37, indicating that it was clear of the main line at Thingley junction.

That's just TWELVE MINUTES. 

It stikes me that there's a lot of - err - incomplete stories around on this topic. On Saturdays early in January, the line from Chippenham towards London was closed for engineering works and First were running an hourly train from Bristol to Chippenham, and turning it around there.  The Wessex train was replaced by a bus (again!) all the way from Westbury to Swindon.  Why?   Wessex trains told me:

I can advise that, unfortunately, there is not the capacity for us to operate a train service along this route as allocation has already been provided to First Great Western. As First Great Western occupies this track for almost 40 minutes in every hour there is not the space for us to also operate between Westbury and Chippenham.