In recent years, there has been a proliferation of research on the effects of the built environment, including mass transit systems, on health-related outcomes. While there is general agreement that the built environment affects travel choices and physical activity, it remains unclear how much of a public health benefit (in dollars) can be derived from land use policies that support walking, biking, and transit. In the present study, we develop a model to assess the potential cost savings in public health that will be realized from the investment in a new light rail transit system in Charlotte, NC. Relying on estimates of future riders, area obesity rates, and the effects of public transit on physical activity (daily walking to and from the transit stations), we simulated the potential yearly public health cost savings associated with this infrastructure investment. Our results indicate that investing in light rail is associated with a 9-year cumulative public health cost savings of dollars 12.6 million. While these results suggest that there is a sizable public health benefit associated with the adoption of light rail, they also indicate that the effects are relatively small compared to the costs associated with constructing and operating such systems. These findings suggest that planning efforts that focus solely on the health impact of modifications in the built environment are likely to overstate the economic benefits. Public health benefits should be considered along with broader environmental health benefits.