The locomotives were built and delivered by the Romanian plant "23 August" (now FAUR) - Bucharest under technical conditions of BDZ.

Maximum interchangeability of aggregates, assemblies and parts with the class 76 machines, built in the same plant, was pursued.

The main differences with them are: All class 77 locomotives are assigned to the Septemvri depot.

There they were sold to serve a narrow-gauge railroad between the coal mines in the Rio Turbio village and the Rio Galegos port, about 200km in the Santa Cruz area of Patagonia.

One of the remaining 5 locomotives was scrapped and the rest continue to work on the Septemvri–Dobrinishte narrow-gauge line.