Some seven thousand buses of the RT family were built for London Transport between 1939 and 1954 creating the largest standardised bus fleet in the world, although a number of design improvements and variations evolved as production progressed. The majority were finished in Central Area red livery, although many were painted green for Country Area and Green Line use.

Most were were based on the AEC Regent MkIII chassis built at Southall, being fitted with AEC 6-cylinder 9.6 litre engines. Transmission was through a four speed air operated pre-selective gearbox with fluid flywheel.

The were built to pre-world war II legal specification. So, they were less than 26 feet long and 7 foot 6 inches wide. They carried 56 seated passengers (30 upstairs and 26 below) and 5 standing. This made then ideal for London's narrow streets. The AEC versions were very quick off the mark and were very popular with crews.

THe RTL (RTLeyland) came about due to high demand for new vehicles after World War II. Initially the Body Builders could not meet the demand and later the chassis builders had a similar problem. There were 1631 RTL's built based on the Leyland Titan chassis but modified to LT requirements. This meant altering the radiator to be the same size as the standard RT and fitting the wilson pre-select gearbox and full air brakes. The chassis was altered to accommodate the RT body. Initially the RTL's were built by Metro Cammell but of course all bodies were interchangable. So, after second overhauls they ended up with different bodies. Many were fitted with early roof box bodies on their last overhaul. The Layland version was not so popular with crews are they were slower and lacked pullong power.

The majority of the standard RT's were bodied by Park Royal Vehicles or Weymann.

The standard type proved immensely efficient and reliable, and some remained in public service with London Transport until as late as 1979. RT 624 being the last in passenger service with London Transport.