A total of six 0-4-0TT steam locomotives were produced by George England and Co between 1863 and 1867 for the Ffestiniog Railway. Two designs were created, four of which were "Small Englands" which featured side tanks and two were "Large Englands" which instead had saddle tanks. The locomotives are designated "0-4-0TT" due to them having both tanks and a tender, with water carried in the tanks and coal in the tender. The locomotives had a low centre of gravity and are extremely small, to help fit in the restricted loading gauge of the line.
(Information provided via Wikipedia)