I don't find this a very compelling cause. After all the passenger service is half hourly, and so there's no problem there with congestion. Unless the line becomes much more busy with freight from Southampton then I can't see the point, when there are many more congested stations (eg Cheltenham) that could really use a third platform.
Of course, if the third platform had a turnback facility then this would completely change the viability of the service that could be provided on the Trans-Wilts with a single unit, but I doubt whether that is the motivation for the enhancement.
One of the problems in the Chippenham area at the moment is that there is nowhere to sidetrack a train that's waiting to go down the TransWilts, so that a freight or passenger service arriving from the Swindon direction and going towards Westbury blocks everything else if there's something already on the way up from Westbury - a nightmare scenario if things get delayed and there happen to be two trains about. It's perhaps partly for this reason that
FGW▸ have chosen to run the morning Swindon to Southampton train even before the first London - Bristol service, and it's why - at current sparse track levels - it would be very hard to use the Chippenham - Trowbridge section anything like intensively. An extra track at Chippenham, providing a "down loop", would sort this out.
The current platforms at Chippenham (I think both of them) can already be used to reverse trains coming from Westbury, so a new track would not
necessarily need the facility ... if a 125 could call at the new face.
There are huge benefits in being able to reverse TransWilts services at Chippenham - at the appropriate (Wilts CC evaluation, not mine) hourly service, it would reduce the number of trains needed from 3 to 2 for a service that goes to Salisbury at the other end, and so reduce running costs by 700k per annum. There is already plenty of seating on 125s between Chippenham and Swindon, and sufficiently frequent a service for good connections to be routinely offered, and that 700k is the difference between it being viable and not.