[1] The first P1, No 2393, was completed by Doncaster in June 1925 and was shown at the Darlington Centenary celebrations in July, with its sister No 2394 following in November 1925.

In service, the P1s could handle 100-wagon coal trains, although these were an operational hazard due to their overhanging track circuits and being too long for many of the passing loops.

The steam pipes from the boiler to the booster were prone to fracture, particularly on the sharp curves of the turning triangle at the New England shed.

2394 was experimentally tried on the 07:45 am semi-fast passenger train from Kings Cross to Peterborough[6] as part of the planning stage for P2 class locomotive 2001 Cock o' the North.

Although the engine was able to reach a maximum speed of 65 mph (105 km/h), this put too much stress on the fireman who later commented that he was thankful not to have been going beyond Peterborough.

[7] In 1942, both engines were overhauled and their original boilers, then in very poor condition, were replaced with A3-type 220 lbf/in2 (1.52 MPa) boilers and their cylinder diameter was reduced to 19 inches (483 mm), leading to an increased tractive effort of 42,500 lbf (189 kN) The valve motion was retained in its original short-lap travel configuration.

The six-wheeled tenders were modified with standard dragboxes to run behind new Thompson Class B2 4-6-0s Nos 2815 and 1632, while the boilers were removed and fitted to Gresley A1s No.