A drive train of a vehicle is disclosed which comprises a permanently driven primary axle and a secondary axle connectable to the primary axle via a switch-on device with a switch-on mechanism, whereby the secondary drive train output via side shaft couplings is transferred to the drive wheels of the secondary axle. Because the switch-on mechanism and/or the side shaft couplings have frictionally engaged couplings, it can be that when the secondary axle is uncoupled, the shutdown section of the secondary drive train located between the switch-on device and the side shaft couplings is uncoupled both from the primary axle and from the secondary drive wheels, such that there is no more power loss in this shutdown section, whereby connecting the secondary drive train during travel is possible all the same and losses in comfort and practicability do not have to be tolerated.