A tractor unit for transporting an elongated load such as a wind turbine blade or wind turbine tower segment, comprising: a tractor unit body; a wheel set comprising only three ground-engaging wheels defining a triangle when viewed in plan view, all of which are driven, said wheel set supporting, driving and steering the tractor unit body; and a load engagement mechanism for mounting to and supporting said elongate load, said load engagement mechanism being mounted on said tractor unit body; wherein the three wheels of the wheel set are steerable wheels whose orientation may be varied to transform the wheel set between an aligned mode of operation and a carousel mode of operation; wherein in the aligned mode of operation the wheels (18, 20, 22) of the wheel set are aligned with one another when in a neutral steering position causing the tractor unit body to follow a straight line when driven with neutral steering, and are steerable to deviate the direction of travel to either side of said straight line; wherein in the carousel mode of operation the wheels of the wheel set are oriented relative to one another with their axes of rotation intersecting at a substantially common vertical axis located within the lateral extent of the tractor unit body, whereby when the one or more driven wheels are driven in said carousel mode of operation, the tractor unit body rotates about said common vertical axis and said common vertical axis is at the incentre of said triangle; wherein said load engagement mechanism is mounted pivotally on the tractor unit body about said common vertical axis, such that the tractor unit body may rotate in said carousel mode of operation about the common vertical axis relative to the ground while the load engagement mechanism remains in fixed position relative to the ground by counter-rotation of the load engagement mechanism relative to the tractor unit body as the tractor unit body rotates. for transporting an elongated load such as a wind turbine blade or wind turbine tower segment, comprising: a tractor unit body; a wheel set comprising only three ground-engaging wheels defining a triangle when viewed in plan view, all of which are driven, said wheel set supporting, driving and steering the tractor unit body; and a load engagement mechanism a tractor unit body; a wheel set comprising only three ground-engaging wheels defining a triangle when viewed in plan view, all of which are driven, said wheel set supporting, driving and steering the tractor unit body; and a load engagement mechanism for mounting to and supporting said elongate load, said load engagement mechanism being mounted on said tractor unit body; wherein the three wheels of the wheel set are steerable wheels whose orientation may be varied to transform the wheel set between an aligned mode of operation and a carousel mode of operation; wherein in the aligned mode of operation the wheels (18, 20, 22) of the wheel set are aligned with one another when in a neutral steering position causing the tractor unit body to follow a straight line when driven with neutral steering, and are steerable to deviate the direction of travel to either side of said straight line; wherein in the carousel mode of operation the wheels of the wheel set are oriented relative to one another with their axes of rotation intersecting at a substantially common vertical axis located within the lateral extent of the tractor unit body, whereby when the one or more driven wheels are driven in said carousel mode of operation, the tractor unit body rotates about said common vertical axis and said common vertical axis is at the incentre of said triangle; wherein said load engagement mechanism is mounted pivotally on the tractor unit body about said common vertical axis, such that the tractor unit body may rotate in said carousel mode of operation about the common vertical axis relative to the ground while the load engagement mechanism remains in fixed position relative to the ground by counter-rotation of the load engagement mechanism relative to the tractor unit body as the tractor unit body rotates. wherein the three wheels of the wheel set are steerable wheels whose orientation may be varied to transform the wheel set between an aligned mode of operation and a carousel mode of operation; wherein in the aligned mode of operation the wheels (18, 20, 22) of the wheel set are aligned with one another when in a neutral steering position causing the tractor unit body to follow a straight line when driven with neutral steering, and are steerable to deviate the direction of travel to either side of said straight line; wherein in the carousel mode of operation the wheels of the wheel set are oriented relative to one another with their axes of rotation intersecting at a substantially common vertical axis located within the lateral extent of the tractor unit body, whereby when the one or more driven wheels are driven in said carousel mode of operation, the tractor unit body rotates about said common vertical axis and said common vertical axis is at the incentre of said triangle; wherein said load engagement mechanism is mounted pivotally on the tractor unit body about said common vertical axis, such that the tractor unit body may rotate in said carousel mode of operation about the common vertical axis relative to the ground while the load engagement mechanism remains in fixed position relative to the ground by counter-rotation of the load engagement mechanism relative to the tractor unit body as the tractor unit body rotates.