continue the sealing surface horizontally toward the front of the cab when the expansion wall section 40 is in its closed position the top plate 95 is joined to the front rear and side sections to complete the main fixed structure of the cab which is then enclosed on all sides except along the outward longitudinal side of the cab which is closed by the expansion wall section 40 to be described next the top section 50 is inclined slightly toward the front of the cab as indicated in fig3 and 8 the expansion wall section 40 as shown in fig1 16 is substantially trapezoidal in shape in its vertical plane to generally conform to the outline of the outward longitudinal side of the fixed cab structure previously described so as to close the opening in this outward side of the cab this expansion wall section is hinged at the front outer edge of the cab to swing in a horizontal plane through an angle x fig2 from a closed position as indicated in fig2 to an expanded or open position providing additional room for the operator at the rear of the cab as indicated in fig6 the expansion wall section 40 comprises a vertical side frame 100 a horizontally sliding access door 102 with a window 104 an upper cover 106 a lower bottom enclosure 108 a vertical.