Soviet Railways series AA 20

In order to create a locomotive with the largest possible boiler without using an articulated running gear with flexible steam lines, the Soviet Railways designed a locomotive with seven coupled axles in a single frame. Originally designed with a 2-14-4 wheel arrangement, it actually became a 4-14-4 due to the axle load that was limited to 20 tonnes. The single prototype was built in 1934 at Luhansk and designated AA 20.

All seven driving axles had a total wheelbase of 10,050 mm. To create some flexibility in curves, the first one had 27 mm of lateral play and the last one had 35 mm. Additionally, the third, fourth and fifth driving axles had flangeless wheels. The coupling rods were created in several sections to create a certain degree of flexibility. The boiler had a stoker and a combustion chamber. In the result, it created an indicated output of 3,700 hp, what was no outstanding value for a boiler of this sheer size.

Hauling capacity was 2,000 tonnes on a one percent incline at 40 km/h. Even the special layout of the drivers inflicted damage to the rails, and another problem was that the full tender actually exceeded the axle load of 20 tonnes. At the same time it was decided to increase the allowed axle load to 23 tonnes, so no more locomotives with seven driving axles were needed. The AA 20 was stored at Shcherbinka and scrapped around 1960.