The principle of the design was to produce a powerful, free-steaming engine to haul the fastest and heaviest express trains on the Great Northern.

These had boilers producing saturated steam at a pressure of 175 lbf/in2 (1,210 kPa) and two outside cylinders, having a diameter of 18+3⁄4 in (480 mm) and a stroke of 24 in (610 mm) using simple expansion driving the rear coupled wheels and fed through slide valves.

The engine worked as a two-cylinder simple on starting, changing over to compound expansion automatically.

A superheater was fitted in 1914, and the engine was rebuilt as a two-cylinder simple in 1917; the new cylinders were outside, 20 by 26 in (510 by 660 mm) of the type used on class H3, driving the leading coupled wheels.

1452–61 built at Doncaster in 1910, had boilers producing superheated steam at 150 lbf/in2 (1,000 kPa), and the cylinders were fed through piston valves.

279 was rebuilt in 1915 with four cylinders 15 by 26 in (380 by 660 mm) utilising simple expansion and driving the rear coupled axle.

1419 (renumbered 4419 in May 1924) was equipped with a booster engine on the trailing axle in July 1923; to accommodate this, the frames were lengthened at the rear, which also allowed a larger cab to be fitted.

They were often called upon to take over trains from failed Pacifics and put up some remarkable performances with loads far in excess of those they were designed to haul.

On 26 November 1950 she hauled a train one way from Kings Cross to Doncaster to mark the end of the C1s.

On display at Doncaster was pioneer sister ex GNR 251, already preserved, and a number of modern engines.

She joined preserved sister GNR 990 Henry Oakley on two weekends of trips entitled Plant Centenarian in 1953, celebrating the 100th anniversary of the Doncaster Works.

[18] On 20 September the two engines, 990 leading, hauled the train from Kings Cross to Doncaster carrying nearly 500.

LNER Class A4 2509 Silver Link (BR 60014) brought the train back to London.

251 steamed poorly on these trips, because the superheater had been removed although the boiler flues had not been replaced with small tubes to compensate.

3287, were discovered at a factory at Essex in 1986 by Steve Dymond and Nick Pigott, the revelation that the boiler once belonged to No.