The Budd Rail Diesel Car, RDC, Budd car or Buddliner is a self-propelled diesel multiple unit (DMU) railcar. Between 1949 and 1962, 398 RDCs were built by the Budd Company of Philadelphia, Pennsylvania, United States. The cars were primarily adopted for passenger service in rural areas with low traffic density or in short-haul commuter service, and were less expensive to operate in this context than a traditional diesel locomotive-drawn train with coaches. The cars could be used singly or coupled together in train sets and controlled from the cab of the front unit. The RDC was one of the few DMU trains to achieve commercial success in North America. RDC trains were an early example of self-contained diesel multiple unit trains, an arrangement now in common use by railways all over the world.

Budd RDCs were sold to operators in North America, South America, Asia, and Australia. They saw extensive use in the Northeast United States, both on branch lines and in commuter service. As passenger service declined in the United States the RDC was often the last surviving conveyor of passengers on a particular route. Most RDCs were retired by the 1980s. In Canada, RDCs have remained in continuous use since their introduction in the 1950s. The RDC inspired several derivatives, including the unsuccessful Budd SPV-2000. The New York Central Railroad strapped two jet engines to an RDC in 1966 and set a United States speed record of 184 mph (296 km/h), although this experimental configuration was never used in regular service.