• In 1874, the Midland Railway imported the first Pullman sleeper carriages from the US. Travellers slept parallel to the direction of travel, in curtained-off cubicles.
• Separate cabins were introduced in the 1890s. Passengers slept crosswise to the direction of travel - as is still the case today.
From
https://www.theguardian.com/uk-news/2019/apr/27/london-scotland-revamped-train-sleeper-romance-revivalI've only ever slept in a crosswise sleeper or couchette, but my sleeper experience isn't anywhere near as extensive as some. I have seen longwise sleepers on Russian trains, what they call "hard class" I think – triple-decker bunks in an open carriage – but, assuming something more akin to a compartment, what are the advantages and disadvantages of sleeping in one direction or the other? I can imagine there might be some difference in how you'd feel the motion of the coach, but I'm not sure what it would be.