The Atlantic Coast Line was one of the few US railroads which used Pacific locomotives for dual service. The P-5-B was the successor of the P-5-A that had been of the USRA Light Pacific design. The P-5-B had a driver diameter of only 69 instead of 73 inches to be used in freight service. In the case of the ACL these were often light and fast trains with agricultural products.
The weight on the drivers could be changed by pins which in turn increased or decreased the weight on the leading truck and trailing axle. This changed the maximum axle load by nearly 4,000 pounds. The newer batches also had a booster. Of 165 locomotives built, the first 70 had Baker valve gear and the rest had Walschaerts. They were retired between 1950 and 1953.