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.

In 1938, the Atlantic Coast Line had Baldwin build twelve 4-8-4 Class R-1 locomotives, also known as Class 1800 because of their numbers, for express train service between Richmond, Virginia and Jacksonville, Florida. Their design was considered very successful and the boiler in particular exceeded many expectations. They pulled 1,500-ton trains of 20 cars and were even able to accelerate them significantly faster than the manufacturer had initially calculated.

Of note, the direct heating area of the actual firebox was increased from 272 square feet to 568 square feet  using a combustion chamber and thermic syphons. The back pressure of the cylinders was also very low, especially in the upper speed range, and allowed greater power output. The only known problem concerned the mass balancing, which was initially calculated incorrectly by Baldwin and still had to be improved.

After the adjustments to the mass balancing, the locomotives were approved for 90 mph and are said to have regularly reached 100 mph. When they later had to pull express freight trains, the maximum train load was set at 6,200 tons. Their service ended between 1951 and 1952.