Vendors come and go quickly on the 16th Street Mall as they discover that rents outweigh earnings. Sure, there's a hot dog stand every other block, two popcorn purveyors, a baked-goods venue and a bento-box spot -- but the pickings often seem very slim. That's when it's time to head over to the Philadelphia Filly, where Sally Rock and Dale Goin, who used to own the Philadelphia Filly on South Pearl Street, have taken it to the streets. In summer they serve up cheese-gooey, steak-packed cheesesteaks; in winter, steamy, flavorful soups. While Goin expedites -- "I need a dubba dubba and a du jour," he barks, calling for a cheesesteak with double meat and double cheese and one with mushroom and green pepper -- Rock rocks on the grill and out of the crock, with all of the food coming quick and fast. Just look for the old-fashioned quilted-metal diner-looking cart, and be prepared to eat your filly for about five bucks.