Complex sentence: Meanwhile the unpowered carriages, when returning from Weymouth to Bournemouth, had the locomotive remain at the rear, pushing them; and a separate driving compartment at what was then the front of the first carriage was used to control the train.
Split into simpler sentences: Meanwhile the unpowered carriages, when returning from Weymouth to Bournemouth, had the locomotive remain at the rear, pushing them. A separate driving compartment at what was then the front of the first carriage was used to control the train.