Abstract: A tractor unit is disclosed for transporting an elongated load such as a wind turbine blade or wind turbine tower segment. The tractor unit body is supported, driven and steered by a wheel set comprising at least three ground-engaging wheels. At least two of the wheels are steerable and can transform between an aligned mode of operation and a carousel mode of operation. In the aligned mode, the wheels are aligned with one another in a neutral steering position, and steerable to vary the direction of steering. In the carousel mode, the wheels of the wheel set are oriented with their axes of rotation intersecting at a substantially common vertical axis located within the lateral extent of the tractor unit body, so that when the wheels are driven the tractor unit spins about this common vertical axis. A load engagement mechanism is mounted pivotally on the tractor unit body about the common vertical axis.