This chapter describes air-conditioning and heating systems for buses, rail cars, and fixed-guideway vehicles that transport large numbers of people, often in crowded conditions. Air-conditioning systems for these vehicles generally use commercial components, but are packaged specifically for each application, often integral with the styling. Weight, envelope, power consumption, maintainability, and reliability are important factors. Power sources may be electrical (ac or dc), engine crankshaft, compressed air, or hydraulic. These sources are often limited, variable, and interruptible. Characteristics specific to each application are discussed in the following sections. Design aspects common to all mass-transit HVAC systems include passenger comfort (ventilation, thermal comfort, air quality, expectation) and thermal load analysis (passenger dynamic metabolic rate, solar loading, infiltration, multiple climates, vehicle velocity, and, in urban applications, rapid interior load change). Ventilation and Thermal Comfort Thermal Load Analysis Bus Air Conditioning Rail Car Air Conditioning Fixed-Guideway Vehicle Air Conditioning