A stack axle assembly as used in a cask transporter or an horizontal storage module (HSM) gantry transporter. The stack axle assemblies include a slew gear; an upper bearing; a lower bearing; a vertical cylinder retained by the slew gear, upper bearing and lower bearing; an independent driver wheel and idler wheel revolving on a common shaft/trunnion assembly attached to the bottom of the vertical cylinder; a frame assembly formed on top of the vertical cylinder, the frame assembly including one or more drive motors connecting an upper drive assembly downwardly to a driven assembly mounted between the idler wheel and driver wheel; and a slew driver motor to drive the slew gear.