The tuna hoagie is the Philadelphia long-roll reading of tuna salad, and its defining quality is that it stays resolutely cold. Where the New England tuna grinder expects the oven and a cap of melted cheese, the hoagie does not go near heat. It is bound tuna on a split Italian roll, dressed the way every Philadelphia hoagie is dressed: shredded lettuce, tomato, raw onion, and a thread of oil. The tuna takes the structural slot the cured meats occupy in an Italian hoagie, and the rest of the build follows the city's standing hoagie grammar unchanged. That refusal of heat is the whole distinction, and it is what separates this from the grinder it otherwise resembles.
The craft is in the bind and the dress working as one cold system. The tuna salad lives or dies on its ratio of mayonnaise to fish and on enough crunch and acid, celery, a little onion, sometimes pickle, to keep it from going to paste, because cold there is no melting cheese to rescue a dull filling the way the grinder gets. The hoagie roll is the structural decision: a long Italian roll with a crust firm enough to carry a wet, cool filling the full length without folding, and a tender interior that does not fight a soft salad. It is split and layered the length of the roll so every bite holds the whole sandwich. The Philadelphia dressing is a deliberate system rather than a garnish: shredded lettuce and sliced tomato add cold and crunch, raw onion supplies a sharp note, and a thread of oil seasons and lubricates without soaking the crumb the way a heavier sauce would. Vinegar and oregano show up in some shops, the same hoagie seasoning logic the meat version uses. It is built fast and eaten cold, before the tomato and the dressing start to weep into the roll.
The variations are mostly the heat decision and the regional name. The same tuna run through an oven with cheese on top is the New England grinder, a different sandwich that shares a filling; the tuna melt takes the cooked logic onto sliced bread; the cold lunch-counter tuna salad sandwich is the same bind on soft bread without the long-roll architecture. It sits in the broad sub, hoagie, hero, and grinder family, one structure under many city names, where the roll is the engineering and the filling is the variable. Each of those is its own build and deserves its own article rather than being crowded in here.