 At the same time, there was an enormous increase in the transportation required in the Erzgebirge mountains as a result of new uranium mines opened by 
"SDAG Wismut".In 1952, in order to assist with the resulting shortage of locomotives, a similar follow-on class emerged, DR Class 99.77-79, built by the "VEB Lokomotivbau Karl Marx" in Babelsberg.At the end of the 1960s, the first Class 99.73-76 engines had to be retired due to boiler damage.