A transport assembly configured to be attached to downhole apparatus has a support assembly configured to be attached to the downhole apparatus and a ground engaging assembly, which may be a roller wheel configured to contact with and move in relation to a surface of a wellbore. The roller wheel may engage with the support assembly so as to provide for their relative rotation. The transport assembly also includes a retaining member disposed between the roller wheel and the support assembly, and the retaining member is operable to resist separation of the roller wheel from the support assembly. The transport assembly is further configured such that, when the transport assembly is in use moving along a wellbore, the roller wheel bears against the retaining member, which in turn bears against the support assembly. The retaining member is a ball-bearing.