It might have been said already, but the Boston areas most notable regional fast food item is the “Nortshore Roast Beef”, ordered as a “Beef three way”. Seems like it’s picked up notoriety, as I’ve seen places popping up in California, Florida, and throughout Southern New England. It’s really messy, and really good.. Pile of roast beef made and sliced in-house, a mess load of this acidic slightly spicy slightly sweet red sauce (like a less vinegary Carolina bbq sauce), white American, mayonnaise, on an onion or sesame seed bun. Fries on the side are seemingly always extra fried, often crinkle cut.
This place does them on Saturdays only:
https://themodernbutchershop.com/
There are dozens of places that exclusively serve them all day every day across Northern Massachusetts. Everyone has their favorite.
For regional chains, Deangelos was a big one back a decade or two. Papa Gino’s too.