The Metropolitan Line of the London Underground, which opened in 1863, was in need of steam locomotives that were fitted with a condensation device to reduce the amount of steam emitted. Since initially three-rail track was laid, the line could initially be operated by the broad-gauge metropolitan class of the GWR. However, since the GWR ceased operations after just a few months, standard-gauge Great Northern locomotives had to step in spontaneously.

In search of their own type of locomotive, the Metropolitan Railway received an offer from Beyer Peacock for tank locomotives with a 4-4-0T wheel arrangement, which should be available within six months and were offered at a price of £2,600 each. They were based on a type that had been developed for the Spanish railway from Bilbao to Tudela. Further orders for identical locomotives came from the LNWR, LSWR and the Midland Railway.

Modernized version with driver's cab

Locomotive Magazine, December 1896

Between 1864 and 1870, 40 examples were initially built for the Metropolitan Railway, which were later to be designated Class A. They did not have a driver's cab, as this would have led to heat build-up in the conditions when driving through the tubes. Almost all of the exhaust steam from the cylinders was fed into the water tanks, preheating the feed water. Initially coke and later high quality South Wales coal were used as fuel, while firemen were trained to produce as little smoke as possible.

Between 1879 and 1885, 26 more of the later class B followed. It had identical cylinders and wheel dimensions, but slightly higher boiler pressure and larger reserves. The wheelbase was reduced and the bogie was no longer in a bissel frame, but was hung up like an Adams axle. These measures improved running characteristics and weight distribution.

Conversion to tender locomotive, Cambrian Railways

Locomotive Magazine, February 1916

As early as 1880, modifications were made to the existing locomotives. These included higher boiler pressures of 150 psi instead of 125 and 130 psi, respectively, 17.5 inch diameter cylinders instead of 16 inch, and 5 foot 10 inch diameter drivers. Driver's cabs were fitted from 1895, even if these were not well received by the crews. The firing of oil was tested on one engine, but not pursued further due to the high price of low-smoke oil.

When the Metropolitan Line was electrified in 1905 and 1906, the surplus locomotives were sold. They reached different parts of the country and were used as shunting locomotives or in front of construction trains. Some pieces even survived into the forties. Of the six units that the Cambrian Railways had acquired used, two were converted into tender locomotives.