A motor drive assembly for selective rail or track operation of an excavators or other mobile platform via a source of hydraulic pressure. The motor drive assembly may include one or more drive arm assemblies mounted at ends of the mobile platform underbody, with the drive arm assembly including a motor, a gearbox, and hydraulic features for operation, including a counterbalance valve assembly, as well as improved gearbox and coupler elements operable therewith. The counterbalance valve assembly may include a cross-port relief valve and may have a first hydraulic operating circuit and a second hydraulic operating circuit. The first hydraulic operating circuit may communicate hydraulic pressure to the gearbox to enable movement of the mobile platform, and the second hydraulic operating circuit may prevent undesired communication of hydraulic pressure to the gearbox.