I am not sure populations has much to do with it. More a lack of infrastructure and technology. Trowbridge and Chippenham are served by alternative services. Melksham doesn't have an IET▸ stop board and the trains are not programmed for a stop there. Both are clearly achievable but with the railway as safety conscious as it is (thank goodness) it is not as straightforward as it perhaps could be.
Westbury and Castle Cary are also served by alternative services, of course, but still the expresses call there when the London trains are cancelled - perhaps that's the answer - that alternative trains are provided if a service to London, where the civil servants who run the railway, are based?
What is going on at the moment is the drawing up of a cancellation/delay mitigation strategy. This involves GWR▸ , Network Rail and TransWilts. It will involve bespoke alerts so TW social media can be updated, better signposting at stations, re-briefing of control staff where there has been quite a bit of turnover and other measures too.
Don't have a precise timetable - things always seem to take longer in the summer with people on staggered holidays. However it is in process.
That is good to read - I hope it is effective, rather than just window dressing. It leads to other questions such as
* Is what is being done going to reduce the number of cancellations and delays to industry standard levels in the first place?
* Where there are GWR or
CRP▸ staff are on other duties at stations, will they be alerted and offering proactive help too?
* Will people be directed to the local buses (and they accept the tickets) when things go wrong?
* Will stop boards be installed and IETs be able to stop after their next software update?
and if answers to those questions are positive, it might lead to a long overdue levelling up for the line.
and less positively
* Why has it taken so many years of problems to have a mitigation strategy?