I've been there before. Amazing station!

I believe (i could be wrong in any of what I'm about to say) that one of the reasons for the splitting and joining of the 444 units is that the section of electrification from Bournemouth to Weymouth was done very cheaply, and so it only supports one unit per train. This means the rear unit of the 10 car train terminates at Bournemouth and reverses in the C.S. to form the front unit for the next Up train. Prior to electrification west of Bournemouth, diesels had to be attached to EMUs for the leg to Weymouth, but the locos could be controlled from the cab of the EMUs too in a push-pull system to prevent the need for running round. Special units existed which were actually unpowered and only used in this way, after being detected from the rest of the train (which was electrically powered). Class 73s also existed which were bi-mode DC and diesel locos, but I'm sure to what extent they were used on this line. Long story short, Bournemouth has always been a bit of an unusual station. "The mainstay of push-pull operations was the operation over the then un-electrified track from Bournemouth to Weymouth and the service continued like this across three decades. Weymouth trains started at London Waterloo powered by third-rail electric traction via Winchester and Southampton to Bournemouth. The consist was normally twelve cars made up of a powerful 3,200 hp (2,400 kW) 4REP electric multiple unit on the rear with two leading units of un-powered 4TCs." "Although class 73 and 74 electro-diesel locomotives had push-pull capability, they did not have the endurance for longer runs on diesel power - their 600/650HP was not even Type 1 - little more than a high-powered shunter equivalent. " WIKEPEDIA

A nice opportunity to view this video

Great railway video

Great video