To use the metro you pay for entrance only, so once you got in (in fact, enter any station) you can go as far and as many time as you wish. Moscow metro is a huge system, and you easily can spend hours roaming through its enormous facilities. That is why it sounds reasonable to have some eatery within the ticket barriers, though the idea wasn't realized till lately. Nevertheless, some year back fastfood spots were introduced at several metro stations (most popular ones, I believe). The one of them is presented in this picture. The quality of food is questionable, of course, but at least it's always within your grasp and you don't have to pay extra money for re-entering the station.
