In the years following the Second World War, the RSR needed a locomotive powerful enough to move its goods trains over the southern mountains and light enough to run on the hastily-repaired track that still made up a large proportion of the route mileage. The newly-appointed chief engineer, Mikhail Rodnivacek, looked for inspiration to Bulgaria, where the only other 4-10-0s running in Europe at that time were doing very good work. Although slimmer-looking, the RSR's 4-10-0s were heavier and more powerful than their Bulgarian counterparts. The 533Bs were allocated almost entirely to the south, where their small wheels and 3-cylinder drive equipped them very well for the steeply-graded lines there. The clipper-sided tender and semi-rigid drawbar enabled them to run equally well in both directions. Of the ninety-five built, some fifty were rebuilt at Akohniçe from 1955 onwards with tapered boilers (with larger grates and combustion chambers), boxpok wheels and Vanderbilt tenders (which made them less convenient to drive tender-first), becoming class 533BB (see below), to which all the remaining survivors belonged. The last original 533B was withdrawn in 1983.

In common with various other RSR designs of the 1950s, some 533BBs were also fitted during 1955 with poppet valves to improve the steam distribution at the high piston speeds that resulted from the small driving wheels. Faster running was certainly possible but caused severe dislocation of the running gear. The poppet valve engines were therefore used for slower work in the southern highlands, where the gain in power could be put to good use.

Graphics and text © Norman Clubb 2015