It would make more sense, it would also allow for a more frequent timetable.
Whoever runs it will need rolling stock and line capacity.
The work done by
SWR» earlier this year (looking at handling the Yeovil to Bristol flow) for the
DfT» as part of their franchise (4 other cross border routes looked at too) has gone into some detail. Rolling stock can be somewhat found by releasing resource that's inefficient in its current use in Wiltshire, added to which a second local (MetroWest) train service into Westbury in each hour gives timing options. Whether the trains are St Phillips Marsh or Salisbury based (so SWR or
GWR▸ ) is academic in terms of frequency though, as neither depot has a thin air machine ...
There is a trend towards clock face services these days, and (with rolling stock) you could run an hourly Bristol to Yeovil service and an hourly Yeovil to Weymouth service. Slight problem being that you couldn't join them up with just a couple of minutes for every train at Yeovil while it let passengers off and on. A train every 90 minutes might be possible ... but by the time you're doing that you're going to move some existing services so much as to adversely effect people's daily lives and actually thin out rather than thicken the service in peak flow directions - such as
reducing the number of trains into Weymouth on a summer weekend morning.