The type was widely used in the United Kingdom and it was also successful in export markets, with numerous examples shipped to Australia, Ireland, India, Spain, South Africa and many other countries.

When the London General Omnibus Company, part of the Underground Electric Railways of London Group, proposed a closed-top double-decker to the group's vehicle-building subsidiary the Associated Equipment Company in 1923, the model 405 was produced, forming General's NS class; this had a frame with side members upswept over the axle mounting points so that the lower saloon floor level was about one foot lower than the preceding types 301 (K-class) and 401 (S-class).

However, the Metropolitan Police, who then had the statutory responsibility for London bus construction and safety rules, refused the fitting of covered top decks on the NS class for a number of years after its introduction.

[1] In 1924 Maudslay of Coventry also introduced a swept-down chassis frame on a comprehensive range of purpose-built passenger models called the ML series, although no double-deckers were catalogued until 1930.

Although the majority of tramways were owned by local councils, some were in the private sector; and although sale of electricity to domestic consumers was beginning to develop as a profit centre, the inflexibility of operation made providing new or extended routes expensive, and on-road boarding began to be seen as obstructive of other road traffic and increasingly dangerous.

Other builders, AEC in particular (at the time in a collaboration deal with Daimler Company), followed suit, getting the drawing offices to stretch existing buses into three-axle versions.

Rackham, under Green's direction, evolved a range of fast, relatively light chassis with powerful engines and a trademark of frames gracefully swept with elegantly varying side-member depth.

The drive-line featured subtle inclination of engine and transmission, allowing straight drive shafts into the underslung worm-wheel single-reduction rear axle – which had the differential offset to the offside to reduce gangway floor height on the lower deck.

[10] Due to their unreliability, extra cost in maintenance and petrol and inherent inflexibility, with both tyre-scrub and driveline wind-up being endemic problems, the big petrol six-wheelers had generally been seen-off by the Titan, Guy was saved by War Department contracts and trolleybus orders, but Karrier went into liquidation, eventually being bought from its receivers by Rootes Group, who used the 6X4 design experience to re-focus Karrier as a trolleybus maker, moving production from Huddersfield to the Commer works in Luton.

Also the Road Traffic Act 1930 had been enacted; as well as promoting consolidation between operators, for the first time maximum dimensions and laden weight were standard for buses and commercial vehicles across Great Britain and Northern Ireland.

[11] Produced from mid-1933, the TD3, like the contemporary Tiger TS6, had a redesigned, more compact front-end layout, which saved 6 inches (150 mm) in engine and cab length, thus allowing coachbuilders to add an extra row of seats within the 26-foot body.

[13] From 1937, a 24-volt electrical system and the oil-engine, both previously optional, became standard, the frame had greater depth over the front wheel arches and a new 7.4-litre push-rod overhead-valve (OHV) petrol engine, known within Leyland as the Mark III, was built for Bournemouth and Eastbourne.

The largest market was in New South Wales, Australia with the Department of Government Transport taking ninety of the 93 exported there during 1946/7, with H33/28R bodies, half by Clyde Coachbuilders and the rest by Commonwealth Engineering.

This new power unit was named after its displacement in cubic inches (US technical influence during World War II led to a standardisation in the British heavy-vehicle market on Imperial dimensions[citation needed] until the late 1960s).

The first production PD2/1 complete with Leyland body went to Todmorden Joint Omnibus Committee, in July 1947 and another early example went to the Northern Ireland Road Transport Board.

The type was mechanically identical to the PD2/4, but to accommodate the body the frame had a re-shaped nearside chassis longitudinal to reduce step height on the air-powered two-leaf sliding entrance doorway, and no down-sweep after the rearmost spring hanger.

It had a large-displacement engine running under less stress, with other innovative features being an air-pressure system, not only working the brakes, but also the change-speed pedal on the pre-selective gearbox, which was built by AEC to Self-Changing Gears and Daimler Company patents.

As in the past, not only with buses, but also with trolleybuses, LT tended to give production of standard types to AEC and work with Leyland on experimental or innovative types, thus 500 of the Leyland order had Leyland-built 8 ft (2.4 m) wide bodies, which, initially, LT were going to class as RTL1-500, so the first of the 7 ft 6 in (2.29 m) wide version entered service as RTL501, some months before the RTWs, as London decided to re-classify the wide-bodied all-Leyland versions.

Another rare variant was the PD2/9, which was almost identical to the PD2/10, save that the chassis was modified to accept a lower-profile central-gangway double-deck body, built to around 14 ft (4.3 m) high, about 6 in lower than normal for a "highbridge" design.

These were to have four-leaf electrically powered platform doors and other new features, the most important of which cosmetically was that they were to conform to BMMO's own-build double decks since 1946 (and most of Birmingham Corporation's since 1949) in having a full width engine-bonnet concealing the radiator.

UTF930 on the other hand had both the tin front and the Orion Body; both were successful on their demonstration tours and later operated for a further 15 years with first Scout Motor Services of Preston and then Yorkshire Woollen District Traction Company of Dewsbury.

"[35] That said they outlasted the contemporary Alexander-bodied Guy Arab IVs delivered in smaller quantities and some ran over 20 years in service, with the late survivors running for Lothian Regional Transport.

Leyland used ECT fuel returns in the late 1950s in its advertising; "Scots find them thrifty" was the headline over a greyscale plate of one, beside a large balloon caption which read "9.75 MPG!

[37] Customers for the narrower PD2/22 version included Jersey Motor Transport, West Riding Automobile Company and the corporations of Darwen, Great Yarmouth, Luton and St Helens.

Having had a good reception at home and overseas for demonstrators with the Pneumocyclic gearbox – which built on the previous pre-selective epicyclic design by being of direct-acting semi-automatic engagement, thus removing the need for a change-speed pedal, and being adapted for fully automatic control, although, being air-pressure operated, it required an air-pressure braking system – Leyland launched four further variants to the tin-front Titan in 1954, two of which were specifically designed to suit the contemporary vogue for lightweight construction (all were air-braked): Early rewards were substantial orders for Glasgow Corporation Transport, which was beginning the programme to replace its extensive tramway system, the much-loved "caurs".

Notable among these were large batches of OPD2/9s for Bombay Electric Supply and Transport Company, previously a Daimler user, and subsequently standardised on Titans to this day.

British construction and use regulations were further relaxed in July 1956, with the maximum double-deck length on two axles being increased to 30 ft (9.1 m) and gross vehicle weight to 14 tons.

During 1960 a new full-width bonnet was introduced, made of glass-reinforced plastic and with a sculpted nearside edge to improve kerb visibility for the driver; this became known as the "St Helen's Front" after its first customer.

This was designed at Albion but production versions were assembled in Glasgow from CKD kits supplied by Leyland; it used the front-end structure of the PD3A almost unmodified but had a complex swept-down frame allowing a step-less forward entry.

In 1941, LPTB were allocated ten 'unfrozen' TD7 Titans (STD 101-11), which received early Ministry of Supply Utility Style bodies by Park Royal Vehicles.