Not a chain, but if anyoen finds themselves in Back Bay Amtrak station in Boston, there's an awesome authentic taco/burrito cart right near the entrance to the station--you walk right by it going to the platforms and the ticket booth. Excellent tacos (beef/chicken/pastor) made with 2 soft corn tortillas (no, real tacos do not have fried shells!), nice blend of flavors, hint of cilantro, etc. Better than anything I've had in a while except maybe for the local Marianne's Tacqueria chain in Boston.