Transportation workers, who constitute a large sector of the workforce, have worksite factors that harm their health. Worksite wellness programs must target this at-risk population. Although physical activity is often a component of worksite wellness logic models, we consider it the cornerstone for improving the health of mass transit employees. Program theory was based on in-person interviews and focus groups of employees. We identified 4 short-term outcome categories, which provided a chain of responses based on the program activities that should lead to the desired end results. This logic model may have significant public health impact, because it can serve as a framework for other US mass transit districts and worksite populations that face similar barriers to wellness, including truck drivers, railroad employees, and pilots. The objective of this article is to discuss the development of a logic model for a physical activity–based mass-transit employee wellness program by describing the target population, program theory, the components of the logic model, and the process of its development.