The railway also commissioned the Beyer, Peacock and Company to construct an additional locomotive of the design for the Pennsylvania Railroad.

[1] The design featured a boiler pressed to 175 lbf/in2 (1.21 MPa) delivering saturated steam to two outside 14-inch (356 mm) high-pressure cylinders, which exhausted to one 30-inch (762 mm) low-pressure cylinder inside the frames.

They were a development of Webb's Experiment class; they had larger boilers and smaller driving wheels, and while the Joy valve gear for the HP and LP cylinders could still be independently adjusted, it was now also possible to reverse both sets simultaneously.

The inside valve gear was subsequently amended to the loose or slip-eccentric system, thus giving automatic reversal.

[2] When George Whale become chief mechanical engineer of the LNWR in 1903, he started a programme of eliminating Webb's over-complicated duplex compound locomotives.