[Split from this thread: http://www.chowhound.com/topics/477982]
The only things that come close are all in RI:
(1) Taqueria Pacifica - LA-style taco truck which now operates out of a shop (although I think the truck might still have scheduled routes); I think there are also various other taco trucks roving the city,
(2) Hot Dog cart by Chez Pascal - hot dogs a la Shake Shack,
(3) Veggie sushi bike cart - Never spotted it, though I think it was parking at the old Decatur Lounge,
(4) Haven Bros. - Classic RI street/truck food.
Has Providence become a center for street food? Sorry, no help with Boston and I guess I'll get deleted or moved.