//Poole is between Dorchester and Bournemouth so must be east of Portland, not west?//
Of course it is, mush. Thanks for the correction and apologies for the duff info. I blame Peter for getting me all confused! :-)
Your memories of the strange rail service to Weymouth are interesting. The line from London Waterloo to the South West was electrified to Pirbright Junction before the war. The juice was extended to Bournemouth in 1967. After then the service to Weymouth was undertaken by twelve coach trains. The front eight were trailers (i.e. unpowered). These were made up of two four car sets with each set having a driving cab at each end like a conventional electric multiple unit. The rear four coach set was powered sufficiently to propel all the twelve. The train would be driven from London from the cab in the front trailer, which controlled the powered four coaches at the rear. At Bournemouth the front four would be split from the train and a diesel locomotive - equipped for "push-pull" operation would be attached to the front to pull them forward to Weymouth. The four electric powered coaches and the four remaining trailers would wait at Bournemouth for the arrival of the incoming service from Weymouth. This would be pushed by a diesel locomotive which was controlled by a driving cab on the front of the first coach. This arrangement meant the locos pulling the trains from Bournemouth to Weymouth did not have to run round their trains before making the return trip. The two portions would be connected, the diesel loco detached and the entire twelve coach train would make its way to London under electric power. Sometimes, depending on demand, eight coaches would be pulled forward to Weymouth leaving the four powered coaches at Bournemouth. The line was finally electrified to Weymouth in 1988.
The reversing arrangement at Dorchester you speak of came about because Dorchester (South, as it now is) was originally built as a terminus station. The line to Weymouth diverged a little to the east of the platforms. This meant trains had to adopt the reversing procedure you describe. Eventually a curved platform was provided for the Weymouth bound services but London services from Weymouth still had to back into the station, an arrangement which prevailed until 1970, when a platform was built for the trains.