A bus (also known as a trailer bus) is a trailer vehicle designed specifically for the transportation of passengers (a bus). Trailer buses typically comprise one of two forms:

Although semi-trailer buses are now typically obsolete and have been retired, full trailer buses attached to conventional buses remain in use in limited applications around the world, including as tourist vehicles.

In 1922 a 65-seater passenger trailer bus went into service for the Department of Parks and Boulevard of Detroit, which was developed by Fruehauf Trailer Corporation, also from Detroit. The bus was 29 ft long, made up of steel chassis and cabin built of natural oak with tram like interior. For comfort the trailer had leaf spring suspension, air vents and four doors for easy access, the doors were interconnected with brakes and were controlled via the tractor unit hauling the trailer. The tractor was a six wheeled Packard tractor unit which was coupled with the trailer via a fifth wheel, making the turning radius as small as 39ft.

An early trailer bus was designed in Amsterdam in the 1920s as bus designs got longer. As a solution to possible grounding hazards on humped bridges, three prototypes were built in 1924, but proved to be problematic, and later converted to rigid bodies in 1927.

In 1930 Nairn Transport Company which ran 550 miles of transport services from Damascus to Baghdad experimented with a trailer bus which was hauled by a Buick Roadster which had a very successful run motivated by this the company later built a customized double-decker trailer bus in 1934 which had carry capacity of 40 passengers divided in first class and normal class. The bus also had reclining seats, an onboard kitchen and a luggage capacity of approximately 700kgs. The trailer was hauled by a Marion-Herrington 6x6 tractor powered by a 188hp Hercules engine, which helped it cover the route in 24 hours. At that time, it was the longest bus ever made with a length of 80ft and weighed 26 tons, making it one of the first successful trailer buses.

During World War II and in the immediate post-war years, trailer buses were turned to as a simple and economical way of providing bus transport to replace worn out conventional bus fleets. The semi-trailers were basic and uncomfortable, but each could carry more passengers than an ordinary single-decker bus, and nearly as many as a double-decker bus. In India BEST of Mumbai and BMTC of Bangalore had double-decker trailer buses in its fleet during the 1970s and 1980s.

In Australia, 123 semi-trailer type buses were built from 1939. Large purchasers included Parramatta-Ryde Bus Service and Rover Coaches. A 1947 semi-trailer coupled with an American-built 1943 White M3A1 tractor is preserved at the Sydney Bus Museum in Sydney. The Sydney exhibit was the last trailer bus used in NSW, withdrawn in 1977.

Trailer buses were also used in Perth, Western Australia by several private bus companies from 1943 onwards, including Metro Buses (formerly Metropolitan Omnibus), Pioneer Omnibus and Scarborough Bus Services. The units were bodied by various companies, including Boltons, Campbell & Mannix and Motor Body Builders, a subsidiary of Scarborough Bus Services. From 1958 onwards, many trailer buses passed to the state-owned Metropolitan Transport Trust as the private operators were bought by the Western Australian Government. A number were also purchased by the Western Australian Government Railways from 1945 to 1948, with the trailer bodies built by Fowler Constructions and Campbell & Mannix.