As the conductor collects fares while the bus is moving, the time spent waiting at stops is reduced – perfect for busy London streets.

Ironically, the first few models looked quite different to the Routemaster we are used to seeing today. Originally expected to last less than 20 years, the fact that the buses were so adaptable meant they kept evolving. Different versions were tried until a design was settled on.

Albert Arthur Molteno Durrent was chief engineer of London Transport's Bus and Coach Division, and created the RT model in 1939 - a double-decker hop-on hop-off bus. It was to replace trolley buses and trams and took engineering to a new level by combining style with passenger comfort.

During a stint as director of tank research and development during world war two, Durrent came into contact with the latest break-throughs in aircraft production, such as using lighter metals and interchangeable parts.

Durrent then began to explore how these techniques could be used in making buses.

The RT was completely redesigned using assembled parts to make the building of them faster and cheaper.

However, London Transport was constantly looking ahead to new possibilities in bus design.

The new Routemaster RM was introduced to London in 1956 and its design was advanced for the time. It was to be a collaborative effort between AEC (Associated Equipment Ltd) and Park Royal, who would fashion the bodywork. These companies worked with London Transport who brought in industrial design consultant Douglas Scott.

By using a light all-aluminium body for the RM, it was possible to produce a 64-seat bus within the weight limits of the older 56-seat RT bus. More passengers could be carried and in more comfort, to try and entice the growing number of car owners onto public transport.

The RM also differed from other London designs because it was made of integral construction. Instead of having a traditional body and chassis, the Routemaster has a strengthened body that does not require a chassis to take the stress.

The mechanical units are mounted on front and rear sub-frames rather than a heavy fixed chassis. Other novel features for a bus in the 1950s, but fitted as standard to the Routemaster, included independent front suspension, power steering, fully automatic gearbox and power hydraulic-breaking.

In 1961, 24 of these vehicles were built and then lengthened by the insertion of an additional 2ft 6in bay in the centre of their bodywork. Called RML, this 72-seat version became the standard product from 1965 and remains the most common type in London today. Weighing about seven and three quarter tonnes, the RML is still two tonnes lighter than modern double-deck vehicles that hold the same number of passengers.

Lots of variations took place during the production of the fleet. During 1962/1963, 575 Routemasters were fitted with Leyland engines from new. From 1964, several batches of vehicles were built with offside illuminated advert panels. Other design changes affected, among other things, the radiator grille, headlamp panels, heater grille and upper deck front windows.

Sixty-eight similar vehicles were built for Green Line routes from 1968. This production batch, classified RMC (Routemaster Coach), were built as 57-seaters, with fully enclosed platforms, electrically-operated doors, air suspension, fluorescent lighting, different interior trim, luggage racks and twin headlamps.

In 1965, 43 further Green Line coaches were built to the longer (RML) 30ft length and called the RCL.

Different adjustments have continued to be made through the decades – a further variation to the standard Routemaster happened in 1996 with Scania engines being fitted to London Centrals RM fleet for route 36.