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. All three cylinders had a stroke of 24 inches (610 mm); the high-pressure cylinders drove the rear wheels, while the low-pressure drove the leading driving wheels. As the two pairs of driving wheels were not connected, the locomotives were "duplex drive" or "double-singles".
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.
