The U is a 0-6-2T narrow gauge locomotive built in larger numbers for several lines in Austria. The first ones were ordered by the Murtalbahn in 1894 that was operated by the kkStB. They were essentially an enlarged variant of the number 1 to 6 of the Steytalbahn. A Krauss-Helmholtz bogie connected the trailing axle with the rear driving axle, resulting in a rigid wheelbase of only 1,200 mm. Maximum train weight on the level was 515 tonnes and 90 tonnes could be hauled on two percent with 20 km/h.

In the next years, several railways in the Austrian part of the empire ordered the same model. Most were built for a gauge of 760 mm, but some in the North also for 750 mm. Production only stopped in 1922 when much more modern locomotives had been available. The Uv of the NÖLB was a variant of the U with compound engine. After a single superheated Uh ordered by the NÖLB, the BBÖ also ordered several Uh in the late twenties.

After World War I, a number of locomotives with both gauges stayed in Czechoslovakia and were designated U 37. Others were now located in Poland, Italy or Yugoslavia. The locomotives which were still in Austria became ÖBB class 298 in 1953. In Czechoslovakia, the first ones were replaced by diesel power in the late fifties and by 1964, all were withdrawn from regular service in this country. Today, some can still be found either on display or in working order with several preserved railways.