Some suggested leaving five driving wheel pairs on the locomotive and only increasing the load from the axle on the rails, while strengthening the railway track.

Both groups of specialists did not take into account that at that time the cars were mainly equipped with a screw harness ( automatic couplers on Soviet railways would be installed en masse only in 1934), which could withstand a force not exceeding 20 tf.

[citation needed] The technical assignment, issued in the spring of 1930, for an alternative preliminary design of a Soviet high-speed freight steam locomotive, with more than five moving axles in one rigid frame, provided for a maximum axial load on the rails of 20 tons, that is, the same as for the preliminary design of a Soviet steam locomotive type 1-5-1 (which would later become the class FD), which significantly complicated the task for designers.

If for the high-speed American type 2-6-1 with an average design axle load from the driving wheel pair on the rail - 26.9 tons - back in 1926, it was possible to successfully implement the specified traction and speed characteristics, then for the high-speed Soviet design version, taking into account given axial load - 20 tons, the number of driving axles had to be increased to seven... At the same time, inevitably, according to the conditions for ensuring fit into the curves, The design of the driving mechanism and crew part became even more complicated.

However, during the detailed design it turned out that when using the 1-7-2 wheel formula it would not be possible to withstand the weight restrictions, so it was decided to switch to the 2-7-2 type.

The locomotive received a radial boiler firebox, a timber frame and a steam superheater of the Chusov system.

The new locomotive received the letter designation AA (in honor of Andrei Andreevich Andreev, who held the post of People's Commissar of the NKPS in 1931-1935) and the full designation of the locomotive AA20-1 (20 is the load from the moving wheelsets on the rails in tons).

The locomotive was also distinguished by an original system of force transmission from the steam engine piston rod to the driving wheelsets.