Recent pledges by the U.S. federal government to increase investments in high-speed passenger rail have highlighted the importance of identifying the potential impacts of these investments on current rail passengers. Identification of these impacts depends largely on the demographics, travel habits, and attitudes of these passengersin essence, who these passengers are. This investigation applied cluster analysis, a method typically used to determine potential consumer groups for marketing research, to identify similar groups of rail passengers riding Amtrak's Hiawatha service between Chicago, Illinois, and Milwaukee, Wisconsin. These groups were then analyzed for their potential responses to future high-speed rail service. Analysis results showed that clustering was an effective method to determine differences between segments of a market, in this case, current passenger rail users. Further analysis indicated that these segments exhibited potentially different responses to future high-speed rail service. It is recommended that attention be paid to the potential needs of these current passenger segments to increase the likelihood of success for future high-speed rail service.