As Paul7755 comments, One of the big problem with slip carriages was that they were unidirectional - great from Paddington dropping a carriage at Little Smuttering on the way to Easton_super_stallion, but not so good on the way back .... the slip carriage couldn't accelerate from the station after the return train had sped by, catch up and couple up. There are, of course, modern times where this sort of unidirectional problem still applies ... Plusbus is great for a ticket from Paddington via Swindon to the Dog and Duck near Washpool. But get onto the bus towards Swindon at the Dog and Duck, and I doubt that the driver will be able to sell you a plusbus ticket to Paddington.
We've been able to do that with sensibly designed regional DMUs▸ ...
Yes, but they then stop to uncouple, and the throughput is very limited. I was on a five car train from Weymouth (158 + 150 + 153) splitting at Westbury in late August, and a decision was taken at around Castle Cary to send the front forward to Swindon and the rear to Bristol, when over 400 people had got on at Weymouth thinking it was the other way ... push chairs, adult wheelchair or two, hundreds (literally) of children. Really needed two wide corridors through the train, which on the British loading gauge would have left no space for any seats.