U.S. airports face significant congestion problems, particularly in major metropolitan areas with continued population and economic growth. In addition to growth in air travel demand, frequent short-haul flights on routes of less than 500 mi contribute to airport congestion. The potential for high-speed rail (HSR) to substitute for aviation on these short-haul routes is well documented; however, there is a need to explore how rail can serve in a complementary mode to relieve congestion at airports by providing short-haul services in support of longer-haul airline services. The primary objective of this research project is to examine the role of cooperation between HSR and aviation to improve the aviation system planning process. This study addresses the following key questions: (a) How have airports, airlines, and rail operators cooperated to enable airport–HSR connectivity? (b) What are the service characteristics of airport–HSR connectivity? (c) What are the unique challenges associated with airport–HSR connectivity? (d) How has the demand for air transportation evolved in the presence of airport–HSR connectivity?