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. This allows the load engagement mechanism to be counter-rotated in an equal and opposite amount to the angular rotation of the tractor unit body in carousel mode, keeping the load in fixed position relative to the ground as the tractor unit spins.
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