There already is a four-story park-n-ride bus station at the former Stapleton International Airport, which is undergoing a $4 billion redevelopment by Forest City. That leg of the project ultimately will connect the Stapleton redevelopment, the bus system and air train.The prospects are Pittsburgh-based Air Rail Construction, led by Adtranz DaimlerChrysler--formerly a part of Westinghouse, which has designed and installed the airport's underground train system; Denver East Corridor Rail Group LLC, headed by Denver real estate investor Doug Jones, who has been trying to get backing for an air train for five years; Denver Transit Alliance, headed by Morrison Knudsen Construction and Kiewit Western, which includes a team of engineers and finance partners; and Denver Front Range Railroad Co., led by Marla Sue Gordon of Denver.