This paper reports on a natural experiment which studied groups of commuters who are equivalent in most important demographic respects, and who had equivalent commuting experiences, but were differentially affected by a new and improved mass transit (rail) route to work. These commuters changed a significant aspect of their commute, but did not change other critical aspects of their lives (e.g., the start and end points of the trips, the jobs they go to, the homes they return to), so that commuting changes were not confounded with other life changes. This study also provided the opportunity to experimentally study - and longitudinally follow - the impacts of this major change in travel time and number of mode transfers in a much more controlled fashion than has been possible heretofore. Our study provided a quasi-experimental design with both cross-sectional and longitudinal comparisons, allowing us to look at within subject differences (how responses vary for people whose trip changes) as well as between subject differences (variation among commuters with different home to work trips), and allowing us to compare differences within one time period as well as over time.