The Baltimore and Ohio Railroad's sole Class N-1 steam locomotive, #5600 (named "George H. Emerson"), was the first duplex locomotive and the first 4-4-4-4 locomotive ever built. It was Unique in the fact that it had a water tube firebox, which was much larger then a normal firebox. It was designed and built by the railroad's own shops in 1937.

The rear set of cylinders were placed beside the firebox. This allowed the locomotive's wheelbase to remain the same. The space beside the firebox was hot and dirty, which caused premature cylinder wear, and the placement of the cylinders limited the size of the firebox. These same problems occurred on the PRR Q1, which also placed the rear cylinders by the firebox. It was built by the B&O's own Mount Clare Shops in 1937; however it had problems with the sizes of the cylinders facing the other direction.

In 1943, No. 5600 was retired from service. The locomotive was then cut up for scrap in 1950.

• B & O 5600 at the 1939 New York World's Fair
• Detail, showing the rearward cylinders and gear

• Reed, Brian (June 1972). Pennsylvania Duplexii. Loco Profile 24. Windsor, Berkshire: Profile Publications Limited. pp. 266–267.

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