I work for one at the moment (a franchise store, not a Subway!), and, it requires quite an investment for a start-up, it's more getting the stock into the store and being placed in the right town at the right time at the right place. Easier said than done but not as hard as you may think it to be after reading that short bit!
Subway seems to do well, smaller stores especially - getting it into the town but not too central.. just get it close to where people work that are likely to go there during/after work, and near a popular bus-stop will definitely help.