11 and substantially parallel thereto and the rod upright portions 18 extend at the rearward portion of the cab 10 further the cab 10 itself presents a window opening which is defined by the terminal end 27 of the cab portion 13 and the abutment or surface 28 of the cab roof 11 also an abutment member 29 is suitably affixed to the cab portion 13 and extends at the angle shown in fig1 and also the portion 28 extends at its angle shown in fig1 and along the sides of the opening all for a purpose hereinafter described a window structure 31 is movably supported on the tracks 14 by means of pairs of rollers 32 and 33 and it will be understood that there are a total of four pairs of rollers 32 and 33 which respectively engage the tracks 14 and fig1 shows only one of each of the two pairs the window structure 31 has a glass piece 34 and an endless frame 35 extends around the glass 34 and is shaped as shown in fig1 and extends endless along the endless abutment of the pieces 28 and 29 also the window structure has a bracket 36 which is suitably connected to and extends between the frame 35 and the lower two pair of rollers 32 and the window structure 31 also has a connector 37 which is suitably affixed to and extends from the frame 35 and supports the two upper pair of rollers 33 thus the entire window structure 31 is movably supported on the rods.