By the 1930s the RSR had become the railway hub of Europe, with many major international routes passing over its metals, and saw the need for a more powerful express passenger locomotive to service them. After meeting various needs arising from the forced departure of his predecessor, Tešlov was able to start work, building on the experience gained with his earlier engines. The sound yet forward-looking design of the 344E bears out the success of this approach very well. French influence is readily apparent in both the compound layout and the general outline of the locomotive. Despite the benefits of the highly efficient Chapelon steam circuit, however, the problem of sometimes indifferent steaming continued to plague the 344Es and a comprehensive rebuilding programme was embarked upon, several engines being fitted with new tapered boilers from 1938 onwards and becoming class 344EE. There were also problems with the trailing bogie, which tended to derail on awkward curves. Starting in 1950, all survivors of the Second World War were rebuilt as 4-6-2s (class 344F) with new trailing wheels, which allowed a much freer passage of air to the grate, and a new front end.