My rationale was to create a non-
HST▸ additional service as follows :
New hourly off-peak Chippenham-Worcester Shrub Hill service calling at Corsham, Bath Spa, Bristol Temple Meads, Filton Abbey Wood, Bristol Parkway, Yate, Cam & Dursley, Gloucester, Cheltenham Spa and Ashchurch for Tewkesbury. Some calls are also made at Oldfield Park, Keynsham, Lawrence Hill and Stapleton Road. During the "Bristol peak", services run to/from Westbury/Weymouth rather than Chippenham. The service also improves commuting opportunities between Bristol-Gloucester and Worcester-Cheltenham.
This would also have the benefit of increasing frequencies at stations further up the line towards Gloucester/Cheltenham/Worcester, when combined with other services.
HST's would only call at Corsham when the above service runs to/from Westbury/Weymouth rather than Chippenham. The HST services would run 3 minutes earlier between Bristol Temple Meads-Bath Spa. This timing alteration applies in the Bristol-London direction only.
It is worth noting that the only difference between my position and
FGW▸ 's on the timing is that they came up with a 5 minute figure for the above calculation. I am happy to stick with mine, given that it was worked out by a very reliable source.
Running the non-HST service to/from Westbury/Weymouth during the peak would also have the effect of increasing service provision along that corridor when it is most needed.
Another part of my reasoning was that Corsham would likely require at least limited direct services to/from London.
See link below for full details. The proposal also requires the third platform to be restored at Chippenham.
http://www.raildocuments.org.uk/jan08/stage2.doc
