Thats interesting - on 170s, when travelling eastbound, and calling at South Milford on TPE services to Hull if the service is formed of 2x 170, the Guard can be seen to open a panel on each door on the rearmost coach, and presumably isolate the door.
I guess the 170 fleet must have some with and some without SDO?
We have to do that with 6 car 185 formations if we are due to stop at liver south p/way, Warrington cent, or sesame (chester-le) street. as the platforms can only take 5 cars it is either isolate the rear 3 by the udb (unit door block) switch and despatch from the GOP or lock the carriage doors of the rear coach out of use. With Yarm the rear 3 have to be isolated as you can get barely 3 1/2 on.
As a throwback to my XC days the 09:03 PAD-MAN sat used to be formed of a west coast hst set (8+2) with a Solihull stop at 6 car length which a hst set was too long for but with grandfather rights was allowed to release doors and away the punters went. On introduction of voyagers said wc hst was replaced by 2x220 coupled together. Because of the Solihull stop back 4 locked out of use!!!
Oh, by the way. The hst buffet car on a sat morning did a roaring trade. Excess of £500 between slough and leamington spa. 8 cars.
Voyager, 4 cars. With the embarassing situation of the rear 4 LOOU for the sake of one station stop no one used! full and standing from reading with still the Thames valley stops still to go and people left behind. Shop takes less than £50. That is just 10% for the same train time.
You do the maths. And they call this progress!!!