An idea could be to run the London Paddington - Cheltenham service hourly throughout the day and use the two units which are used on the Swindon - Gloucester/Cheltenham shuttles.
That would, I believe, require two additional
HST▸ sets. There ain't anything spare there.
Rolling stock utilisation for services through Melksham is something the TransWilts Community Rail Partnership has looked at in some depth. Could you remind us of the findings, grahame?
During the day, there are 3 x HSTs on London Paddingon -> Cheltenham and 2 x 15x on Swindon to Cheltenham.
Every hour, a train takes 30 minutes to turn around at Cheltenham
Every 2 hours, a train takes 70 minutes to turn around at Swindon
And there's a 50 minute turn around at Paddington (if it's the same set)
Swindon to Cheltenham takes a few minutes over the hour. So on a line that was a self-contained service that would take 3 trains to provide the hourly departures from each end. However, through trains to London alternating with a more local service rather frustratingly leave the two local units running for less than 60% of the time and sat at the end of the line for over 40%.
Extending local trains from Cheltenham to the South? You can get beyond Chippenham and towards Trowbridge before you had to turn back, but not (robustly) all the way to Trowbridge.
Look now to the south, at Salisbury you have a 15x train terminating and turning around (from Romsey) that's there for 40 minutes in every hour. Not long enough to robustly get to Warminster and back. And you have another 15x train that's there for 28 minutes in every hour, from Waterloo.
And in the middle at Westbury ... oh my goodness, this one is complicated. A train arrives at around :36 from Bristol and one leaves at :38 towards Bristol - clearly not the same unit. Some go on to Frome and some of those to Weymouth. A few head south via Salisbury. Sometimes local shuttles run from Westbury to Warminster to serve Dilton Marsh.
There *are* options available at certain times of day to get better utilisation, but these aren't trivial and you end up with the possibiity of providing an odd service at times that don't make for good, realistic round trip / commute schedules. You've got to watch single lines at Kemble, on the TransWilts, and at Chandler's Ford. You've got to watch interaction with the expresses who's tracks you're using in different parts, freight, long signalling sections etc. And if you do a great deal on linkage, you may end up having the *time* available, but not in the right part of the two hour slot. Add one unit and you end up with many more options; from either Salisbury or Swindon you can extend a train to Swindon or Salisbury and provide a two-hourly TransWilts, even through the single journey is a smidgin over an hour.
The Franchise consultation in January to March asked about franchise boundaries, and there are some oddities in the area south of Westbury - such as Dean and Mottisfont which are operated by First but only have Stagecoach services. I'll be most interested to see what's in the
ITT▸ about this.
All four bidders for the next franchise have timetabling experts that know far more about this kind of stuff, and indeed if there had been an easy answer under current parameters, I suspect it would have been offered. I'm going to defer to them rather than speculate here further