In 1942 rolling stock for transporting heavy freight was built parallel to the development of the class 52 locomotives. This was the origin of the type Ssym 46 six-axle flat car. It had an empty weight of approximately 22 metric tons and a loaded weight of 80 metric tons. The maximum speed for these cars was set at 80 km/h or 50 mph. After the war this class of cars was used to transport construction machinery, machine parts, steel products, concrete parts for construction work and many other types of heavy, single piece loads.