Double-decker bus
The first commercial horse-drawn double-decker omnibuses were introduced in England in 1847 by Adams & Co. of Fairfield, Bow, then improved upon by John Greenwood, who introduced a new double-decker in 1852.
Leyland Titan (front-engined double-decker)
The first of Rackham's new T-Type Leylands was, importantly, the double-deck bus. This was the Titan TD1 (mythological names for double-decks and animal names for single-deckers and lorries grew to be a Leyland tradition); it was a masterly synthesis of worldwide best practice in design. It and the single deck Tiger TS1 equivalent were announced at the London Olympia Motor Show of 1927 after an intense development period.