The Thompson A2/2 Pacifics
On succeeding Gresley,
Thompson started a process
of standardisation. His first proposal concerned a mixed traffic Pacific. This was basically a
mixed traffic non-streamlined A4 with Walschaerts valve gear on the centre
cylinder. This design would replace the
P1,
P2, and
V2 classes, along with some of the
work being performed by the
B7,
B16, and
K3 classes.
Thompson started by rebuilding the
P2s.
These locomotives were built for the line
between Edinburgh and Aberdeen, but the wheelbase was found to be too long for the continuous curves of
this line. Also, the swing-link pony truck was causing damage by adding unnecessary stress to the frames.
A similar pony truck arrangement on the
V2s would eventually by fixed, but
Thompson decided to approach the
problem on the
P2s with a complete rebuild.
Existing parts would be used as much as possible, including the short outside connecting rods.
This led to the outside cylinders being mounted relatively far back. The
Gresley conjugated gear on the middle
cylinder required a high level of maintenance, so
Thompson replaced this with
Walschaerts valve gear. For clearance, this meant the middle cylinder had to drive the leading driving
axle.
A Kylchap exhaust arrangement was fitted, as well as small wing-type smoke lifters.
The first engine diagram appeared in August 1942 and rebuilding occurred
between 1943 and 1944 at Doncaster.
After rebuilding, the six A2/2 locomotives returned to Scotland to work the Edinburgh to Aberdeen
line. At the end of 1949, they moved to England and were divided between the York and New England sheds.
They kept their tablet exchange apparatus during the rebuilds, but this was removed after they were
transferred to England.
Under the 1943 renumbering scheme, they would have been numbered 990-5, but they did not receive their
new numbers until after the 1946 renumbering scheme when they received the numbers 501-6.
They then received the British Railways numbers 60501-6.
Withdrawal occurred between 1959 and 1961. The A2/2s are often considered the least successful of
Thompson's Pacific designs, but
it has to be borne in mind that he was working with
the flawed P2 design.
Technical Details
The following details are for the class as a whole. Initially, No. 2002 carried
the boiler with the long combustion chamber from
P2 No. 2006. This had a firebox of
11ft 9in, with correspondingly shorter boiler tubes. This was scrapped after 18 months.
Details relating to the tenders are for the standard 8-wheel tenders. The A2/2s were initially fitted with
six wheel tenders.
| Cylinders (x3): |
|
20x26in. |
| Motion: |
Outside: |
Walschaerts |
|
Inside: |
Walschaerts |
| Piston Valves: |
|
10in. diameter |
| Boiler: |
Max. Diameter: |
6ft 5in |
|
Pressure: |
225psi |
|
Diagram No.: |
106A |
| Heating Surface: |
Total: |
3132.74 sq.ft. |
|
Firebox: |
237 sq.ft. |
|
Superheater: |
679.67 sq.ft. |
|
Tubes: |
1211.57 sq.ft. (121x 2.25in) |
|
Flues: |
1004.5 sq.ft. (43x 5.25in) |
| Grate Area: |
|
50 sq.ft. |
| Wheels: |
Leading: |
3ft 2in |
|
Coupled: |
6ft 2in |
|
Trailing: |
3ft 8in |
|
Tender: |
4ft 2in |
| Tractive Effort: |
40,318lb |
(@ 85% boiler pressure) |
| Wheelbase: |
Total: |
63ft 0.9in |
|
Engine: |
36ft 11in |
|
Tender: |
16ft |
| Weight (full): |
Total: |
161 tons 17cwt |
|
Engine: |
101 tons 10cwt |
|
Tender: |
60 tons 7cwt |
| Max. Axle Load: |
|
22 tons |
| Water Capacity: |
|
5000 gallons |
| Coal Capacity: |
|
9 tons |
Preservation
The last A2/2 was withdrawn in 1961, with none surviving into preservation.
Models
Apple Green Engines produce a ready-to-run model of the A2/2
in 4mm scale. PDK, DJH, and Millholme sell 4mm scale kits of the A2/2. Crownline have also produced a 4mm scale kit
in the past, but this is no longer available.
DJH also sell a kit of the A2/2 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. 60505 Thane of Fife.
Thank you to Malcolm Peirson for the above picture of No. 60503 Lord President.
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