Yes ATA has significant operations out of MDW, but still less than Southwest. By in large, ATA has been avioding going head to head against Southwest. From MDW they both compete on flights IND, MCO, PHX, LAS, and FLL. (and soon SEA) ATA flies to a few east coast cites that Southwest has stayed away from for congestion reasons (PHL, LGA, DCA, BOS, EWR) and flies to some smaller markets in Florida. Right now they are expanding, but into international markets. They are about to add Grand Caymon, Aruba, Cancun, and (i think) Guadalahara fom MDW. Many new gates are going to be added and it will be interesting to watch how thinks develop. I would be suprised to find out that Southwest has any plans to international anytime soon. They are not close to reaching that "finite limit of places to fly to". WN receives 100s of requests for service from cities every year. Since they add only one or two markets a year, that gives them a lot of time. They have many markets that they have yet to enter (long haul etc.) and they can always gain more market share in the markets that they already fly in.