It's always been common to have none corridor trains on suburban services, and it was only in the 70s when these types of trains started to have internal corridor connections, but even today few have them between units, the 455s and 150/2s being the only examples, but the latter is more than purely a suburban unit, being used on a wide variety of services, even if it is not the suited to them all. Outer suburban units though and those used for medium distance services have for some time all had internal corridor connections, although those between units are still not that common. Express and long distance units and hauled stock has nearly always had corridor connections throughout the whole train, with only a few units not having end corridor connections, these either being long enough to rarely if ever be coupled in normal service (such as the original trans-pennine units, and of cource things like HSTs, Eurostars and Pendolinos, the the former 2 are classed as units in the traditional sence), or have inter-unit corridor connections at one end only, which would normally be in the middle of a coupled pair of units. This has a number of uses, mainly allowing the guard to walk right through the trains for revenue protection or ticket sales or allowing trolley services to go along the length of the train or passengers to walk to fixed buffets. With modern stock though between unit gangways are rare on all types of unit, with the only examples being on the SR Mk1 replacement stock, and since a lot of their work is outer suburban in nature they least require them. No DMU from the 170 onwards though has had Gangways between units, nor have the 350s which see as much is not more Express work as Outer Suburban. This causes problems for revenue protection and onboard services, often requiring crews, often compleate with trolley, to move between units at stations, increasing dwell times, or leaves half of the train without trolley or buffet faciulities for the entire journey, or even without seeing a ticket inspector (great for fare dodgers), or requires two crews, one for each unit, which is costly and can be unreliable (often one half will not have catering staff due to a shortage), or it discourages TOCs from doubling up units, either causing overcrowing on busy services (including at weekends when units often run doubled up due to lower frequencies). I think this will be the case of FTPX, since I'm sure I have read that they don't have enough units to run any doubled up.