The invention relates to a motor vehicle drivetrain, in particular for a commercial vehicle or rail vehicle, having a drive engine, which has a drive shaft and generates an exhaust stream; having a power turbine positioned in the exhaust stream for converting exhaust energy into drive power; the power turbine being switchable into a drive connection with the drive shaft in order to drive the drive shaft or an assembly downstream of the drive shaft in the flow of drive power; having a hydrodynamic retarder, comprising a driven vaned primary wheel and a secondary wheel that is stationary or driven in the opposite direction to the primary wheel, said wheels together forming a working chamber that can be filled with a working medium in order to brake the primary wheel hydrodynamically by torque transmission from the primary wheel to the secondary wheel by means of a working medium circuit. The motor vehicle drivetrain according to the invention is characterised in that a mechanical disconnect clutch is provided, by means of which the power turbine and the primary wheel of the hydrodynamic retarder can be decoupled from the flow of drive power in the motor vehicle drivetrain.