Commonwealth Railways CB class railcar

Rail Diesel Car (RDC)
Former Baltimore and Ohio Railroad Budd RDC-2 at the Danbury Railway Museum
Interior of the RDC-1 demonstrator in 1949
In service	1949–present
Manufacturer	Budd Company
Constructed	1949–1962
Number built	398
Capacity	RDC-1: 90 passengers RDC-2: 70 passengers, baggage section RDC-3: 48 passengers, RPO, baggage section RDC-4: RPO, baggage section RDC-9: 94 passengers
Specifications
Car body construction	Stainless Steel
Car length	RDC-1/2/3/9: 85 ft (25.91 m) RDC-4: 73 ft 10 in (22.50 m)
Width	10 ft 0 3⁄8 in (3.06 m)
Height	14 ft 7 in (4.45 m)
Wheel diameter	33 in (838 mm)
Wheelbase	RDC-1/2/3/9: 68 ft (20.73 m) RDC-4: 56 ft 10 in (17.32 m)
Maximum speed	85 mph (137 km/h)
Weight	113,120 lb (51,310 kg)
Prime mover(s)	RDC-1/2/3/4: 2 × GM 110 diesels RDC-9: 1 × GM 110 diesel
Power output	RDC-1/2/3/4: 550 hp (410 kW) RDC-9: 300 hp (220 kW)
Transmission	Hydraulic torque converter
UIC classification	RDC-1/2/3/4: (1A)(A1) RDC-9: (1A)2′
AAR wheel arrangement	RDC-1/2/3/4: 1A-A1 RDC-9: 1A-2
Braking system(s)	Air
Track gauge	4 ft 8 1⁄2 in (1,435 mm)

The Budd Rail Diesel Car, RDC 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.

The self-propelled railcar was not a new concept in North American railroading. Beginning in the 1880s railroads experimented with steam-powered railcars on branch lines, where the costs of operating a conventional steam locomotive-hauled set of cars was prohibitive. These cars failed for several reasons: the boiler and engine were too heavy, water and fuel took up too much space, and high maintenance costs eliminated whatever advantage was gained from reducing labor costs. In the 1900s steam railcars gave way to gasoline, led by the McKeen Motor Car Company, which produced 152 between 1905–1917.J. G. Brill sold over 300 "railbuses" in the 1920s. Newcomer Electro-Motive Corporation, working with the Winton Motor Carriage Company, dominated the market at the end of the 1920s but had exited it completely by 1932 as the Great Depression gutted rail traffic.

The Budd Company entered the market in 1932, just as EMC exited. Up to that time Budd was primarily an automotive parts subcontractor, but had pioneered working with stainless steel, including the technique of shot welding to join pieces of stainless steel. This permitted the construction of cars which were both light and strong. Budd partnered with Michelin to construct several rubber-tyred stainless steel rail cars powered by gasoline and Diesel engines. These saw service with the Reading Company, Pennsylvania Railroad, and Texas and Pacific Railway. The cars were underpowered, the tires proved prone to blowouts and derailments, and the cars were unsuccessful.