Why B-Units Disappeared: The Truth Behind The Diesel Booster

For a brief period in the early diesel era, American railroads ran locomotives in A-B-A and A-B-B-A configurations — cab-equipped lead units paired with cabless booster B-units that provided traction power without requiring an operating crew. The B-unit seemed like an elegant efficiency: all the horsepower, none of the cab overhead. Then the operational realities caught up. B-units couldn't be turned independently at wyes or loops. They couldn't be serviced without separating the consist. When isolated from the A-unit, they required a separate movement authority and crew anyway. The horsepower they provided could eventually be concentrated into a single higher-powered A-unit. By the late 1960s, the B-unit had quietly lost every practical argument it had once made for its own existence.