River Irt is the oldest working 15-inch gauge locomotive in the world. The chassis is that of the 0-8-0 tank loco Muriel, built by Sir Arthur Heywood in 1894 at his home at Duffield Bank in Derbyshire. He believed the 15 inch gauge was the minimum gauge for serious use on estates and by the army.
Muriel was intended to be the most powerful loco possible on such a gauge and incorporated special features such as radial axles to allow it to traverse a 25-foot radius curve.
In 1900 it was sent to work on the Eaton Railway briefly, then on Heywood's death in 1916 was requisitioned and believed to have been moved to the Gretna munitions factory.
Muriel arrived at Ravenglass in July 1917. The cylinders were reduced in a major overhaul in 1921 as the original launch-type boiler could not produce enough steam for the continuous run.
The loco was given a new locomotive-type boiler in 1927 and at the same time the frames were extended for the rear truck, a small cab and a 6-wheel tender were added in place of the side-tanks. The locomotive was renamed River Irt. Despite the odd appearance of the miniature outline on the wide frames, the loco was a great success and became the mainstay of operations.
In 1972 its appearance was improved by the addition of a taller chimney, cab and dome together with a new bogie tender of narrow-gauge proportions. Its performance was enhanced from 1977 with the fitting of a larger boiler similar to the other Ravenglass steam locos.
River Irt formed the centrepiece of the 1981 Minimum Gauge Railways exhibition at the national Railway Museum in York. In 1984 it operated at the International Garden Festival at Liverpool, and in 1990 at the Gateshead Garden Festival.
The loco currently has a mid-green livery with black and yellow lining.