Travel time savings, better quality of supplied services, greater comfort for users, and improved accessibility are the main factors of success for high-speed rail (HSR) links. In this paper, results are presented from a revealed and stated preference survey concerning HSR and air transport users in the Madrid–Barcelona, Spain, corridor. The data gathered from the stated preference survey were used to calibrate a modal choice model aimed at explaining competition between HSR and air transportation in the corridor. The major findings of the paper describe the demand response to different policy scenarios considering improvements in the level of transport services. From the model, prices and service frequency were found to be among the most important variables in competing with the other mode. In addition, it was found that check-in and security controls at the airport are a crucial variable for users in making their modal choices. Other policies, such as the improvement of parking facilities at train stations, play a secondary role.