A powertrain system for driving a vehicle comprises an engine, a transaxle including a hydraulic motor, a hydraulic pump separated from the hydraulic motor, and a gearbox joined to the hydraulic pump. The engine includes an engine output shaft extended in the lateral direction of the vehicle. The hydraulic pump and the hydraulic motor are fluidly connected to each other so as to constitute a hydrostatic transmission. The gearbox supports a PTO shaft. The gearbox incorporates gears for drivingly connecting the engine output shaft to the hydraulic pump, and for drivingly connecting the engine output shaft to the PTO shaft extended in the fore-and-aft direction of the vehicle.