If I’m not mistaken, one of the largest Subway Franchisees is a yid named Feldman. His Subway Development Corporation headquartered in the DC area owns over 1000 Subways. I have heard that Mr. Feldman is a frum jew who keeps kosher (but I have no way to confirm that, have never tried, and it may be nothing more than a popular urban legend).
If true, there would seem to be two issues with it; doing business in non-kosher food is a problem, and getting benefit from basar b’chalav is an even bigger problem.