BBÖ 729

.After the First World War, with a smaller Austria, the BBÖ wanted to use tank locomotives for express trains in order to save the fees for turning locomotives on foreign turntables for services to border countries. However, the Südbahn 629 series was too weak for these tasks and their fuel and water capacities were too small, so that they had to be supplemented at great expense abroad.

To remedy this, the head of the BBÖ construction department, Alexander Lehner, together with the Lokomotivfabrik Floridsdorf, designed a 2′C2′ h2t (4-6-4, superheated 2-cylinder, tank) locomotive, designated 729 series. The design used the boiler from Südbahn 109 series (BBÖ 209), while the bogies, wheels, drive and valve gear came from the 629 series. The locomotives had Lentz valve gear and Heinl mixer-preheaters.

Six were delivered in 1931 and four more in 1932. In 1936, six copies with Nicholson thermic syphons were reordered, the changed dimensions of which are shown in the table. The BBÖ not only used the locomotives on the originally intended border routes, they also hauled light express trains on the 300 km long western line between Salzburg or Linz and Vienna, such as the Orient Express, the Arlberg-Orient Express and the Ostend-Vienna-Express.^[1] A third order comprising ten locomotives had already been delivered to the Deutsche Reichsbahn, which classified the machines as the 78.6.

The series survived the Second World War without losses and was designated series 78 by the ÖBB. Equipped with a Giesl ejector and front-end throttles, they were in use until 1973.