Why would he mention “linking up to the Salisbury route”, if he was aiming at Castle Cary?
I believe he is talking about the south chord into the GWR goods station at Yeovil Junction which was built when the rest of the line was built but never had track laid on it.
The idea has been around since the '80s to lay track on it and bring the down Island platform at Yeovil in to use to reverse the Westbury to Weymouth service there to provide interchange.
The most recent iteration of this idea involves double track between Dinton and Tisbury so that all Waterloo to Salisbury trains can run to Yeovil and stops can be omitted on Waterloo to Exeter services (ultimately post a Woking flyover with a third hourly London to Salisbury service enabling trains to run non stop from London to Yeovil other than calls at Salisbury and Clapham Junction giving a journey time from London to Yeovil Junction of one hour 40 (about 2h 25 to Exeter Central calling at Crewkerne, Axminster, Honiton and Cranbrook)
At Yeovil Jct the train would divide into Barnstaple or Okehampton (ultimately Plymouth) and Weymouth portions.
Given that trains from Yeovil Pen Mill to Weymouth do it with calls at Maiden and Dorchester West in 38 minutes (28 minutes to Dorchester), it can be seen that with a moderate line speed increase from the current 60 to 80 or 90 between Yeovil and Dorcherster a journey time to Weymouth of 2h 15 and Dorchester of 2h 05 would be doable.
This is the thick end of an hour faster than the current journey time from Weymouth to Waterloo of three hours (2h 50 from Dorchester) and also makes travelling from Dorchester to places like Salisbury and Exeter feasible.
That tends to get the local MPs attention (to be fair one of them might well have hatched the plan in the first place while wearing a different hat).
None of this is greatly different to what Chiltern have already done.