4-14-4

Soviet locomotive class AA20
The only 4-14-4 locomotive ever built
Type and origin Power type Steam Builder Voroshilovgrad Locomotive Factory Build date 1934 Total produced 1
Specifications Configuration: ​  • Whyte 4-14-4 Leading dia. 760 mm (29.92 in) Driver dia. 1,600 mm (62.99 in) Trailing dia. 1,050 mm (41.34 in) Length 33,730 mm (110.66 ft) Axle load 20 t (20 long tons; 22 short tons) Fuel type Coal Firebox:  • Firegrate area 12 m2 (130 sq ft) Boiler pressure 17 kgf/cm2 (1.67 MPa; 242 psi) Superheater: ​  • Heating area 174 m2 (1,870 sq ft) Cylinders Two, outside Cylinder size 740 mm × 810 mm (29.13 in × 31.89 in) bore x stroke
Performance figures Maximum speed 70 km/h (43 mph) Tractive effort 320 kN (71,940 lbf)
Career Operators Russian Railways Locale Russia First run 1935 Scrapped 1960 Disposition Scrapped
[1]

The sole example of this type, called the AA20-1, was built by the Soviet Union. The designation stands for Andrey Andreev (who sponsored its construction), 20 ton axle load. While some builders had produced twelve-coupled (six driving axles) designs, no one had ever built a fourteen-coupled engine. The AA20-1 holds two records: the largest number of coupled axles on a locomotive, and being the longest rigid frame locomotive in Europe. It was the largest rigid frame locomotive in the world until 1939, when the PRR S1 was unveiled.

The large number of driving axles were meant to spread out the locomotive's weight, reducing the axle load and the resulting stress on the track. A group of Soviet locomotive engineers had visited the United States, and presumably they had seen the 4-12-2 locomotives being used by the Union Pacific. The 4-14-4 could be viewed as an expansion of that type. Unlike the successful Union Pacific locomotives, however, the AA20-1 was a complete failure.

The AA20-1 was originally intended to be a 2-14-4 (1'G2'). Construction was undertaken in Essen, Germany by Krupp, following a Soviet design. Later, the unfinished locomotive was moved to Lugansk, where a two-axle leading truck was fitted. The trailing trucks supported a huge firebox. The AA20-1 was intended to run on low-quality coal, so a lot of space was needed to burn it in order to provide enough heat to the boiler.

The seven driving axles were laid out 1.7 m (5.6 ft) apart, giving a very long rigid wheelbase of 10 m (33 ft). This required adaptations to negotiate curves. The center three driving axles were blind, and the first and seventh driving axles were fitted with lateral motion devices. These measures were not enough to make the AA20-1 work properly, however. It was prone to frequent derailments, and its large number of wheels made the engine too heavy for the track. As a result of its long wheelbase, the AA20-1 also spread the track and wrecked the points of the switches as it passed over. It was too big to fit on the turntables, too powerful for the couplers in use at the time and unable to run at full power for very long due to its undersized boiler.

The AA20-1 made a publicity trip to Moscow in 1935. It was then put into storage at the Shcherbinka test facility and finally scrapped in 1960, though this was not stated publicly.

• History of rail transport in Russia
• Russian Railway Museum, Saint Petersburg

• Stalin's Engine: The AA20