There were only open and closed cars in the beginning of railroad freight service, but soon there were special cars for certain types of freight. There are currently still open freight cars with gravity unloading used – so-called dump or self-unloading cars – to transport construction materials such as gravel or sand. Such cars are also used in track construction to distribute ballast over the track. The hatches arranged on the underside are simply opened for unloading, and the load slides out down the chutes. The RhB also uses various versions of such cars.