Italian State Railway classes 746 and 747

When in the early 1920s the existing six-coupled express locomotives of the FS could no longer provide sufficient adhesive weight on hilly lines, the FS was one of the first railways in Europe to develop an eight-coupled express locomotive. This was the only way to comply with the maximum axle load of 17 tonnes that applied on hilly routes lines as those in the Rome and Florence area. It was based on the 2-6-2 locomotive of the class 680 and added another driving axle. 

While up to this point the Plancher system was preferred for four-cylinder compound locomotives in Italy, here the von Borries system, which was also used in Germany, was used. There were thus two inside high-pressure cylinders and two outside low-pressure cylinders, all of which acted on the third driving axle. In order to create space for the connecting rods of the inside cylinders, the cylinders were arranged somewhat higher and inclined, and the second driving axle was slightly cranked.

In contrast to the bar frame, which was very popular with express locomotives on the European mainland at the time, a plate frame with a thickness of 30 mm was used here. The leading axle with a lateral play of 94 mm was combined with the first driving axle to form a Zara bogie and gave it a play of 20 mm. Due to the large grate area, the trailing axle was further away from the last driving axle and therefore had a lateral play of 140 mm.

The production initially consisted of 40 locomotives which were supplied by Breda and had Walschaerts valve gear. Another 20 came from Ansaldo and received a Caprotti valve gear. These were initially designated class 477 and redesignated 746.100 in 1929. After their original routes were electrified, the locomotives were re-stationed once each in 1935 and 1937. They eventually landed in Sicily and were retired by 1967. Today, there are only two preserved members of the class: the 746.031 in Milan, where it was once made, and 746.038 at Pistoia.