The expanded Fenway Frank starts from the same boiled beef frankfurter and the same New England top-split bun, then does the one thing the plain ballpark version refuses to do: it lets the eater build it. This is the whole distinction. The base frank is identical, neutral, held in hot water, designed to be assembled fast and carried far. What changes is that the bun arrives as a platform rather than a finished object, and the toppings, mustard, relish, raw and grilled onion, sauerkraut, whatever the stand carries, become a decision the eater makes instead of a fixed two-item spec dressed by a vendor not looking at it.
The craft is in the bun, and the New England top-split is the reason this build works at all. Unlike a side-split bun, the top-split has two flat, crustless faces that can be lightly griddled into a gold wall on each side, which braces a soft roll against a heavier, wetter load. A side-split bun under a fully dressed frank tears at the hinge and goes to paste in the hand; the top-split holds its shape because the toasted faces do structural work the soft interior cannot. That single piece of bread is what separates a frank you can pile onto from a frank you cannot. The boil still matters for the same reason it matters in the plain version: the frank has to hold serving temperature through a long line, and a held frank stays plump where a grilled one tightens and dries. The difference is entirely in what the top-split lets you stack on top before the structure gives.
The plain two-item ballpark frank, the one that is deliberately restricted to mustard and relish so it can be made identically ten thousand times, is its own sandwich with its own concession-stand discipline and deserves its own article rather than being crowded in here. So do the regional dogs that throw out the build entirely: the Chicago dog with its strict arrangement and its banned ketchup, the coney under a fine beanless sauce, the bacon-wrapped Sonoran loaded like a boat. Each keeps a soft split bun and rebuilds everything above it, which is the same instinct that turns a fixed ballpark frank into a build you finish yourself.