The Thompson A2/3 Pacifics

In 1944, Thompson authorised the building of thirty standard Pacifics based on his A2/2 design (his rebuild of the P2s). The boiler pressure was increased to 250psi, and the cylinder diameter was reduced to 19in. A further thirteen were authorised in 1945, although these and half of the original order were actually built as Peppercorn A2s. A total of fifteen were built to Thompson's design between 1946 and 1947.

These were the first new Pacifics built at Doncaster for eight years. Hence, they were marked by a number of innovations, including steam brakes, a hopper ashpan, electric lighting, and a self-cleaning smokebox. However, the rocker gate and V-shaped cab (as seen on some of Thompson's rebuilds) were absent.

Initially, the design included the small wing smoke deflectors, but these were changed to the large smoke deflectors before any of the A2/3s were built.

At first, the A2/3s worked short distance trains whilst teething problems were fixed. The main problem concerned the steam pipe which ran from the dome to supply steam to the injectors, ejectors, etc in the cab. Compared to the Gresley Pacifics, the boilers were shorter and the steam-operated brakes were much more effective. Hence, braking would cause boiler water to slosh forward and some would be taken into the steam pipe. This could cause the injectors to fly off if they were in use. Also, when the ejectors were next used, they could send the trapped water through the ejector pipe and out the chimney - drenching anyone in the vicinity. This problem was fixed by adding an extension to the steam pipe. This pipe was kept as high as possible, allowing the water to drain back into the boiler. A drain valve was also added to the ejector exhaust pipe. Most of the A2/3s had these modifications fitted from new.

Comparison tests with the V2s were performed. This showed little doubt that the double blastpipe and chimney on the A2/3 was superior to the V2 arrangement, but that the A2/3 smokebox temperature was higher. This latter point resulted in more lost heat through the chimney. The final conclusion was that the A2/3s were wasteful of coal on light runs, and that their greater capacity could only be effectively utilised on heavier duties.

Five of the A2/3s were allocated to the Southern Area, one to Scotland Area, and the remaining nine to the North-East Area. This only changed in 1963 after withdrawals had started to occur, and the remaining locomotives were concentrated away from the North East Area.

The first A2/3s were withdrawn from service in 1962, and the last were withdrawn in 1965.

Technical Details

Preservation

The last A2/3 locomotives were withdrawn in 1965, and none survived into preservation.

Models

Apple Green Engines produce a ready-to-run model of the A2/3 in 4mm scale. PDK and DJH both sell 4mm scale kits of the A2/3. Crownline have also produced a 4mm scale kit, but this is no longer available.

DJH also sell a kit of the A2/3 for O gauge (7mm scale).

Locomotives

Acknowledgements

Thank you to the P.H. Groom collection for permission to use the above photograph of BR No. 60515 Sun Stream.