It may surprise some that, until the 1950s, almost all local service stock was like this, apart from that cascaded down from long distance services, and this last was not too popular operationally because it had toilets which needed facilities to service them, often not available at local service sidings. Just a few emu fleets had been built with open stock, many were compartment as well.
The 1957 Class 501 were indeed Spartan, ironically they had replaced what according to past writers (because they went well before my time) were some of the most comfortable open saloon electric stock there had been, the 1920 LNWR Oerlikon sets, which even got special accolades from general writers like Sir John Betjeman.
Stock with compartments right across was known as "non corridor", to distinguish it from that with a corridor down one side, generally for main line services. Stock without connections between carriages was "non gangwayed". Side corridor stock was generally gangwayed, but there were some which were not. The Southern 2-HAP electric unit (late 1950s) described above was one such. These were "better" than basic suburban stock as they had first class, and were felt to need some provision for toilets, although in only one of the two coaches. Corridors in a coach were principally seen as needed to access toilets, and gangways between coaches to access any restaurant car further along the train.
There was little open main line stock of the current style, especially on the old GWR, and I can still recall BR Mk 1 open stock in the 1960s being incorrectly referred to by older passengers as "restaurant cars". The GWR only built open main line stock, specially, for excursion work. There's a very nice, about 1935, one at Didcot museum, which makes you realise just how comfortable vehicles of that era were, especially given they were built for low-fare summer holiday work (they were also dug out for relief trains at Christmas etc) - and that it's been sat out in the open in sidings for 80 years.
The big advantage of non corridor stock was seating capacity. Theoretically 6 a side (think current 3+2 suburban stock, with an extra seat where the middle walkway is), 12 to a compartment. If 10 compartments in a coach, as in some Southern 4-SUB units, that's 120 seats per coach. When I see modern stock like the 378s with 40 seats along the sides. I think that's one THIRD of the onetime seating capacity. Then you look at all those crushed in standing, and think that in the old stock they would all be sat down.
The downside was finding a seat on a busy train, passengers had to walk along the platform to spot one, extending dwell times. And on outward journeys, as passengers left, you could get some compartments now empty where the next one was still crush loaded. There could also be criminal activities which, although extremely rare, were a part of the sudden move to get rid of such vehicles in a rush around 1980.
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