The Czechoslovak state railway looked attentively at the operating and service costs, and decided to order a three-cylinder single-expansion engine with axle load up to 16 tonnes, which could operate on all main lines. The result didn't exactly meet the original proposal, because of a higher axle load, but the locomotive was great step forward in the construction of steam locomotives in Czechoslovakia.
Design
The frame is of the girder type, cast of vanadium steel. The twin-axle leading bogie has side play of ±, the rear truck of Adams type ±. The centre coupled axle, which is the driving one, has its flange reduced by to allow negotiate curves of radius. The springs of the coupled axles and trailing truck are mounted above the axle boxes and are mutually joined by means of rockers. The three cylinder steam engine has cranks arranged at ±120° and the central cylinder has inclination 1:10. Cylinders have a bore of and stroke of. Pistons and piston valves are made of aluminium and have hollow rods for to keep the weight in desirable limits. The boost compensator was self-acting, Winterthur type. The valve gear is of the Heusinger type, with internal link motion driven by flycrank on the left wheel of the rear coupled axle. This arrangement proved itself very reliable in contrast to maintenance problems with the Gresley conjugated valve gear and was used for all subsequent Czechoslovak three-cylinder locomotives except of one series. The boiler is impressive: the centre line is above the rail and consisted of two rings, in diameter respectively. The cylindrical boiler, which had 158 large flue-tubes and 53 small flue-tubes long, had a heating surface of. In the smokebox, which is long, was located a small tube superheater with a heating surface of. The copper firebox had grate area of and a heating surface of. The steam dome with throttle valve was on the first boiler ring and was cowled together with pneumatically operated sandbox. The safety valve was of Pop-Coale type cowled together on the firebox; the maximal pressure was. The first batch had the vacuum brake and the air-compressed brake for both the locomotive and train as well ; subsequent batches were fitted without vacuum brake. Tachometer was of Hausshälter-Rezsny type, the lighting by means of acetylene, which changed from the second batch to TELOC tachometer and electric lighting. Other changes and improvements were:
From the third batch onwards: there were four arch tubes in the firebox, hence the heating surface got larger to
From the third batch onwards: large tube superheater was used with heating surface.
From the third batch onwards: the cylindrical boiler had 126 large flue-tubes and 35 small flue-tubes with heating surface.
From the third batch onwards: smoke deflectorswere fitted, as seen on the picture.
As the locomotives progressed through the workshops for repairs all of these improvements were retrofitted. After 1945 copper fireboxes were changed for steel ones, there were fitted Houlson grates and the arrangement of cylindrical boiler was also changed and maximum speed was united at. Piston valves were newly of Trofimov type and except of Nos 13, 14, 16, 18, 24, 26, 32, 34, 35, 36, 37, 38, 39 and 43 were Kylchapexhaust system fitted. The locomotive weight is ; maximum speed of was probably because of either length of track sections between semaphores, or safety with variable quality of the track. According to Mr. František Vrátil, former dispatcher in Choceň, the class was able to reach. The class is said to have been 'among the most successful locomotives' in Europe. The locomotives started to be retired in 1967, with last one being retired in 1974. One example, 387.043, has been preserved. Tenders were of series 923.0, 930.0 and 930.1
