How does the comfort of a "flat seat" compare with a compartment?

Trains have fuselage cross-section/loading gauge width of around 300 cm. Slightly less in Great Britain, more in most of the rest of the world (on broad track gauge). This means that trains can definitely be crammed with seats of limited or no recline arranged 4 or 5 abreast, 2-2 or 2-3.

But in upper classes, trains normally have compartments. An aisle to a side of car against wall, and private compartments on the other side, with 2 or 4 or 6 berths.

Logically, a sleeper car can have considerably fewer tickets than an all-seat car. However, the comfort must be worth the price.

Why donīt planes have private compartments? 757 or 320 are slightly wider than railroad cars, and should have the space for compartments. Widebodies ought to allow compartments on both sides of aisle?

What planes in first and business class have is single deeply reclined or lie-flat seats. If lie-flat seats really are such a fabulous idea, why do not trains have them?