Although the 2-10-2s of class 533A were more than the equal of their work, there was till a need for a locomotive for the more lightly-laid routes, in particular the lines from Kropčahne to Tstaie and Lamechov. The addition of an extra driving axle, combined with a more modern, 3-cylinder compound power unit, resulted in this machine, the 634A. One of the few firms still in the steam locomotive business, Babcock & Wilcox got the order. The Babcock engines acquitted themselves very well on the work they were given, regularly hauling loads of 1200 tonnes up the 1 in 85 gradients to Pelašin and Čavoka. By now standard features included boxpok wheels, poppet valves and Vanderbilt tender. At the time of writing, the whole class is still in service, sharing work with the Garratts and 4-10-4s.

The Chinese class QJ 2-10-2, which went into production at Datong Works in 1964, carried a skyline casing. In order not to be left out, Ketterik had a few 634As fitted with this device, which, although it serves a purpose on the QJ (as cladding for the external steam pipe from dome to superheater), is of no use to an RSR locomotive. The casings had disappeared by 1971.