A tractor unit system includes a tractor unit that has a travel drive gear (310). The travel drive gear (310) includes a drive gear portion (315) and at least one drive roller portion (316). The system also includes a guide track (100) that has a track gear (130). The track gear (130) includes a track gear portion (131 ) and at least one land portion (132). The at least one drive roller portion (316) and the at least one land portion (132) are configured such that the at least one drive roller portion (316) contacts the at least one land portion (132) when the drive gear portion (315) is engaged to the track gear portion (131 ). In some embodiments, a diameter of the at least one drive roller portion is equal to a pitch diameter of the drive gear portion. If the track gear portion is curved, in some embodiments a diameter of the at least one land portion equals a pitch diameter of the track gear portion. In other embodiments, if the track gear portion is straight, a dimension of the at least one land portion equals a pitch line of the track gear portion.