After various Mallets with the wheel arrangement 2-6-6-2, the C&O had the class H-7 built with the wheel arrangement 2-8-8-2. In 1923 and 1924, 25 H-7s were delivered by ALCO and in 1926 20 H-7a by Baldwin followed, which had a slightly higher adhesive weight. Since, unlike their predecessors, they worked with simple expansion, they were called “Simple Simons”.

By 1935 they received a boiler pressure of now 215 instead of 205 psi and thermic syphons. The 632 square foot firebox heating area now included 145 square feet from the combustion chamber and 165 from the thermic syphons. They could haul a 9,500 short tons freight train 113 miles in five hours.

They were replaced on the flatter lines by the class T-1 2-10-4s and on hilly lines by the 2-6-6-6 “Alleghenies”. 30 of the 45 locomotives were later leased to the Union Pacific. The Richmond, Fredericksburg & Potomac also bought some locomotives.