A working machine has a main frame structure which includes a pair of endless tracks; each track is entrained about track supporting devices mounted by a track sub-frame. The sub-frame includes inner and outer members interconnected by cross members. The inner and outer members are arranged side by side and mounting at or adjacent upper edges thereof, at least one upper track support device between the members. The inner and outer members mount at or adjacent lower edges thereof, at least one lower track supporting device between the members. The inner and outer members and cross members of at least one of the track sub-frames is a monolithic casting.