Notes: Popularly known as the Erka, the Er (rekonstruirovannyi - redesigned) was a more powerful and economical development of the Em The prototype was Er 723-12, rebuilt from Em at Murom, and about 270 new locomotives duly followed, plus some rebuilds from Eu and Em. New construction was stopped in 1936 in favour of the SO , but war prompted a hybrid emergency batch from Kolomna (series 750) using Su type boilers and SO-type wheelsets and axle-boxes. In 1947-55 some 2682 engines with increased superheating were built for the MPS by East European factories, many as reparations, and several hundred more were delivered for industrial use until 1957, a number of which were taken into MPS stock in the 1960's: examples of the latter include nos 792-64 and 793-75. Thus, about 10,700 of the E family entered MPS stock, with a possible class total of over 11,000 - the world's largest steam class. (Technical data above is for engines from about 1950- for example, from no.768-19 in the MAVAG series.)
