The Fenway Frank is the rare sandwich whose defining quality is the room it is eaten in rather than anything on the plate. It is a boiled or grilled beef frankfurter in a soft bun with yellow mustard and bright green sweet relish, and that spec is deliberately, almost stubbornly, plain. Strip away the ballpark and it is an unremarkable hot dog. Keep the ballpark and it is the only hot dog that tastes like this, because the sandwich was engineered to be assembled by the thousand, carried down a concrete aisle, and eaten with one hand while the other holds nothing. The frank is a fixed, neutral baseline on purpose: nothing about it is supposed to demand attention away from the field.
The craft is in the boil and the constraint. A frank held in hot water rather than seared stays plump and yielding and, more importantly, holds at serving temperature for as long as it takes a vendor to work a section, which a grilled dog does not. The bun is soft and structurally minimal because it has to be split, filled, and wrapped in seconds without a counter, a plate, or a second pass. The toppings are restricted to mustard and relish for the same reason every part of this sandwich is restricted: the build has to be identical the ten-thousandth time, dressed by someone not looking at it, eaten by someone not looking at it either. The acid of the mustard and the sweet-sour cut of the relish are the only flavor decisions, and they exist to keep a salty frank from going flat over the length of an afternoon. This is concession-stand engineering treated as a real discipline, which is exactly what it is.
The loaded build, the one that piles on the toppings and turns the same frank into a full plate in a bun, is its own sandwich with its own logic and deserves its own article rather than being crowded in here. So do its regional relatives that change the rules entirely: the Chicago dog with its forbidden ketchup and its specific arrangement, the coney under a fine beanless meat sauce, the Sonoran wrapped in bacon. Each keeps the soft split bun and rebuilds everything above it, which is the same impulse that let a plain boiled frank earn a name of its own.