Of the two regional poles the tamago sando swings between, the Kanto style is the one most of the world already pictures: a smooth, creamy, near-homogeneous egg salad pressed between crustless shokupan, the form that fills convenience-store cases across Tokyo and the eastern half of the country. Where the western regions reach for a sliced omelette, Kanto keeps faith with the mash. The egg is worked finer here, the whites broken down close to the texture of the yolk, the kewpie mayonnaise folded through until the filling is one continuous pale-gold spread with no visible seams.
That smoothness is the entire technical argument, and it is harder than it looks to get right. Hard-boiled eggs are chopped and then pressed or riced until the distinction between white and yolk nearly disappears, bound with enough kewpie that the mass turns spreadable but stops short of loose. The vinegar-and-yolk sharpness of kewpie keeps a finely mashed filling from going flat, which is the real risk: over-mash without enough acid and it reads as bland paste. A correct Kanto filling is uniform, glossy, and dense, spread wall to wall on soft untoasted shokupan so every bite is the same creamy mouthful from corner to center. The faults are specific to the smooth style. Push the mash too far with too much mayonnaise and it slumps into something closer to dressing than egg; underwork it and you have lumps that betray the homogeneous ideal; cut the bread thin and the soft filling soaks straight through. The bind has to be tight enough to hold two slices and stay cohesive without weeping, because there is no textural rescue here, no chunk of egg to break the monotony if the seasoning is dull.
Within the mashed-egg lineage the Kanto sando is the conservative baseline, and the variations move outward from it rather than from the omelette side. The double-egg build sets this exact salad against a slab of folded omelette; the half-boiled version trades the fully set, finely mashed yolk for a jammy soft-boiled one that runs; the demi-glace treatment keeps the smooth filling but drowns it in a dark Western sauce that flips the register from clean to heavy. Set against its Kansai and Kyoto cousins, where a thick dashimaki turns the whole sandwich eggy and bouncy, the gap is wide enough that the comparison is structural rather than incidental. Each of those divergences runs on its own logic and deserves its own article rather than being crowded in here.