GregL is correct. If you plan to walk from St. Louis' concourse A to C you will leave the security sterile area, and you must go to TSA Security at Concourse C once more. If your time permits, the walk is well worth it and takes a leisurly 30 minutes discounting TSA check-in lines. But there are a lot of things to see. For instance, a replica model aircraft of Charles Lindberg's transatlantic flight, a very extensive mural of the progress of the US Air Force, many shops, restaurants and bars, plus the opportunity to stretch your legs. I would go for it and walk! ^