Throughout the 1950s, the bulk of express passenger traffic was in the hands of Tešlov's 4-6-2 rebuilds and his new 4-8-2s of 1950, which were essentially a passenger version of the 533A. Rodnivacek set out to develop a new engine to serve the RSR for the coming quarter century. The experience gained from the solitary trial balloon, the 434B, was invaluable in optimising the dimensions of the 434C for maximum efficiency and performance. Major aspects of the development were a tapered boiler, boxpok wheels and poppet valves. On test, no. 434C.001 hauled a train of 890 tons gross up Rovniebera bank at 125 km/h, the boiler pressure holding perfectly steady at 18.7 atm. Starting on the bank was also no problem, despite the lack of a booster. Subsequently, a further 49 engines were turned out by the Akohniçe works in 1962 and another 50 in 1963. With its low running and maintenance costs and high availability, the class 434C was, like the later 564B, a first-class investment, and continues to serve the RSR to this day.