B&O No. 3684

The B&O Railroad and General Motors helped pioneer diesel-electric technology in the early 20th century. In 1925, General Motor Electro-Motic Division (GM-EMD) unveiled the CNJ No. 1000, the world’s first commercially available diesel locomotive. The revolutionary switcher was operated by the Central Railroad of New Jersey (CNJ) in conjunction with the B&O Railroad. Then in 1937, the B&O purchased the world’s first diesel-electric passenger locomotive (B&O No. 51) from the same manufacturer.  

The demand for diesel engines steeply rose in the years proceeding WWII. By 1960, the B&O fleet was entirely dieselized (though 44% of the company’s freight revenue came from the transportation of coal). In 1966, GM-EMD the B&O purchased 16 GP-40 diesel-electric engines from GM-EMD. The No. 3684 was the first of those engines to arrive, and the first B&O locomotive equipped with an AC alternator (the successor to the DC generator). The B&O would go on to purchase 380 GP-40s between 1966 and 1981 – more than any other railroad in the nation.  

The B&O GP-40s were the first engines installed with a “rate of change” wheel slip system. Developed by B&O Chief Electric Engineer C.M. Machin, this system was so effective at detecting slips that it was soon adopted by all major locomotive builders. It is still applied to locomotives today.  

The No. 3684 serviced the Westbound No. 397, a mixed freight train that operated between Van Bibber, MD and Wilmington, DE. In the late 1980s, the engine was renumbered as CSX No. 6500 and repainted blue and grey to match the CSX fleet. The No. 6500 was acquired by the B&O Railroad Museum in 1993, and the engine’s original colors and number were restored the following year. A second round of cosmetic restorations were applied in 2006. The No. 3684 is still operational to this day.