The line was closed Chippenham - Swindon on Saturday and Sunday and also Bath - Chippenham on Sunday , with the only trains serving Chippenham being from Westbury (possibly a first!) on Sunday. Even there, a crew shortage on Saturday evening lead to the last round trip from Westbury to Chippenham being at  18:50 rather than 21:25 - so that looks like timetabled service reduced in the plan due to engineeringm then further reduced on the day because of s lack of crew.

Although all stations WERE served,  rail replacement services still needed from Chippenham to Swindon - not realistic to expect anyone but a rail enthusiast to travel a long way round for such a short journey even if there're enough crews and trains. 

When Bath - Chippenham closed, some logic in reversing hourly trains at Bradford Junction and with calls at Melksham and B-o-A (for changes) the local service could be replaced for the day.   When Chippenham - Swindon closed, is there any logic in an hourly London - Swindon - Parkway - Temple Meads - Chippenham - Westbury - London route?  I suspect not - just too far round.   With Chippenham accessible only from Westbury, some sense I'm making that into an hourly shuttle and advising change at Trowbridge for London - Bristol service via the Berks and Hants.    All of which comes down to "what price", "where do you get crew and trains from"?

 
Could the services not run London to Chippenham via Swindon and then terminate at Chiienham and then go back to London

AND

then have a Paddington to Bath via Bristol Parkway and BTM terminate at Bath and then make the return trip via BTM and Bristol Parkway?

No need for replacement services

My suggestion reminds of an airplane story from Australia many years ago

British Airways plane route was London Sydney Melbourne Sydney then back to London
capacity on the Sydney to Melbourne and Melbourne to Sydney was only 20 per cent full
so British Airways decided to change the route to London Sydney Melbourne then onto London
which meant there was nearly 100 per cent capacity on the route from Sydney to Melbourne

 
More likely to be the staff shortage that has reappeared this weekend.

It would need more staff than usually scheduled to run as you suggest & they can't even rustle up the usual number!

 
As mentioned previously, HSS drivers at Paddington and Bristol signed Newbury and Freshford. But so many HSS drivers have retired or reduced hours plus choose to be not available for Sundays as per the terms and conditions.

When I retired the only GWR depot that signed Bedwyn to Westbury was Exeter but I don't think they sign Bathampton to Westbury.

So due to the reduction of HSS FESWs then has it been decided up high not to attempt to run trains from Reading next stop Bath Spa via Newbury?

 
The line between Chippenham and Swindon is closed this weekend for engineering work.

Yesterday, one of the two half hourly Bristol to London services was diverted via Badminton not calling at Bath and Chippenham with the other diverted via the Berks and Hants after calling at Bath.

Today however, Bath Spa has no London services with passengers either having to change at Bristol or Westbury with some long connection times at both.

In previous times, when the line between Chippenham and Swindon is closed, on Sunday Bristol to London trains would start/terminate at Bath Spa to provide a direct service for a station that has a lot of London passengers.

Why is this not happening today? Cost cutting or because it stretches train crew availability? I suspect both.

I expect the local trains between Westbury and Bristol will be very busy today with many diagrammed as just two or three coaches.

Sadly this is another example of not providing a train service at times of engineering work. Westbury and Pewsey having not had a London trains the past three Sundays because the line was closed in the Frome area.