. . . With a combined population of 104,000 in its catchment area, along with providing easy connections to Salisbury, Southampton and Portsmouth, it is not an idea entirely without merit, although personally I wouldnt like to see any stops between Reading and Taunton for the Penzances if a semi fast service were introduced.
Compared to non-stop Taunton - Reading times of between 1:13 and 1:19, a stop at Westbury results in a TAU-RDG journey of 1:22. A single stop at Castle Cary can be included for a TAU-RDG journey of 1:19, so with improvements to the run-in speed at Westbury, possible by removing the avoiding line junctions, a similar TAU-RDG time should be feasible with a single stop there - the cost being 4 minutes worst case. The current inconsistent times for the non-stops may be down to conflicts through the less than optimal Reading interim layout which will be solved once the final grade separation there is accomplished next year. With a standard hourly stopping pattern as far as Plymouth, whether or not the train goes on to Cornwall or not, a completely standard clock-face timetable can be provided at all station en route. I think the stop at Westbury is worth it for the rail interchange connectivity and the site's potential as a regional parkway railhead, especially with the local road access improvements I suggested, possible by using the avoiding line alignment as a town bypass (note the previous proposed southerly road bypass route around the town was rejected a number of years ago).