(03/18/2002) -- Waiting his turn at driving the M-113 armored personnel carrier is STS-110 Mission Specialist Rex Walheim. In the background, right, is Mission Specialist Ellen Ochoa. The driving is part of Terminal Countdown Demonstration Test activities, which include emergency egress training and a simulated launch countdown. The TCDT is held at KSC prior to each Space Shuttle flight. Scheduled for launch April 4, the 11-day mission will feature Shuttle Atlantis docking with the International Space Station (ISS) and delivering the S0 truss, the centerpiece-segment of the primary truss structure that will eventually extend over 300 feet