In 1862 and 1863, the Baltimore & Ohio built eleven ten-wheelers for the increased demand in the Civil War which they designated class B. The first one, numbered 117, had 64 1/2 inch drivers and a combustion chamber that soon had to be removed due to leakage issues. It also had flangeless front drivers. Although they belonged to the same class, the rest had drivers with a diameter of 60 or 58 inches.

The second locomotive was preserved and stored in 1892. For the “Fair of the Iron Horse” in 1927, it was rebuilt, renumbered to 117 and was given the name “Thatcher Perkins”. When the Mount Clare roundhouse in Baltimore collapsed due to excessive snow in 2003, the 117 was one of the locomotives which were heavily damaged. In the meantime, it was cosmetically restored.

The largest class of Consolidations built for the Baltimore & Ohio was the E-27. The intended area of service were heavy freight trains on both flat and mountainous routes. In order to keep the strain on the locomotive and tracks low, a driver diameter was chosen that was larger than on previous consolidations and at the same time a long cylinder stroke of 30 inches with a comparatively small diameter.

205 of the original E-27 were built in 1905 and 1906 at the ALCO plants in Schenectady and Richmond. The E-27b only followed in 1909 and was largely the same. Changes only affected the Walschaert valve gear and two-inch larger wheels. After 84 E-27b had been built, 120 more followed, which had a larger number of thinner diameter tubes in the boiler and were given the same designation.

When being superheated, different variants were created, which were given different class names. The differences here lay in the superheater, the cylinder dimensions and the valve gear. A total of 116 E-27 were rebuilt into class L-2 0-8-0 switchers. The last E-27 were retired in 1959.

Although the Baltimore & Ohio wanted to buy diesel locomotives for freight traffic in the forties, they had to order steam locomotives due to the war restrictions. This was the birth of the class EM-1 with the wheel arrangement 2-8-8-4, called “Yellowstone”, being the last articulated of the B&O.

They were thoroughly modern locomotives with roller bearings on all axles including the tender. Thanks to five thermic syphons, arch tubes and a long combustion chamber, the firebox had an enormous surface of 756 square feet, larger than that of the Big Boy. With 117.5 square feet, the grate was smaller than that of the Big Boy that had to burn low-quality coal. Both lead drivers and the lead axle of the trailing bogie had some sideways play.

At the beginning, they operated in the Cumberland area with challenging grades. Services included everything from express freights to heavy coal trains, with occasional passenger trains. For the heaviest trains, they operated in double with another locomotive pushing at the rear. Later they came into the Pittsburgh area and were scrapped between 1957 and 1960.

The name “Old Maude” refers to a mule in a comic, but also the first large Mallet in the USA built for standard gauge. Its job was as a helper for heavy trains on inclines, what didn't require high speeds. This meant that she did not require any carrying axles, meaning that all six axles were available for traction. The top speed of 21 mph or 34 km/h was completely sufficient for her tasks. Although the locomotive, which was initially designated class O and later renamed DD1, did not have any sisters, it remained in service until 1938.

In 1927, the B&O put a total of 20 examples of the P-7 class into service, which at the time were the most powerful Pacifics ever. The power was achieved with a driver diameter of 80 inches and cylinders measuring 27 by 28 inches. The firebox, with a combustion chamber and thermic syphons, reached an area of 394 square feet. The first locomotive was manufactured with a booster on the trailing axle, the others were only prepared for its later installation. They were given names after the first 20 Presidents of the United States, beginning with No. 5300 “President Washington”.

The P-7 hauled the most important express trains of the B&O. Over time, there was no standard rebuild of the class, but various different modifications. This resulted in the classes P-7a to P-7d, which differed primarily in the heating area and, among other things, had different superheaters. The P-7a and P-7d were also streamlined. Trains they hauled included the “Royal Blue” and the “Cincinnatian”. Their service ended in 1958 after the introduction of diesel locomotives in express service.

Between 1932 and 1936, the Baltimore & Ohio built four different and unique 4-6-4 „Hudsons” which were designated classes V-1 to V-4. What they had in common was the water tube firebox designed by master mechanic Colonel George H. Emerson. After the V-1 had been a rebuild of a P-1c Pacific with an extremely long firebox, the V-2 was built from the ground up in 1935 in the B&O's Mount Clare Shops.

It got the number 2 and was named “Lord Baltimore”. Inspired by the British GWR King class No. 6000 “King George V” that had visited the Fair of the Iron Horse, it was designed with all tubes and rods inside the boiler shell to create a clean look. The boiler itself was operated with an exceptionally high pressure of 350 psi. Large drivers with a diameter of 84 inches and cylinders with a small diameter assured high speeds with light trains.

After No. 2, the B&O built two additional 4-6-4 in its Mount Clare Shops in 1935. No. 2 was used to haul the new lightweight train “Royal Blue” between New York City and Washington, D.C. Later it hauled short express trains over the hilly line between Washington, D.C. and Cumberland, Maryland. It was scrapped in 1949, its sisters a year later.

The Davis Ten-Wheeler was a 4-6-0 Camelback locomotive of the Baltimore & Ohio that was designed by J.C. Davis and stood in the tradition of the locomotives from Ross Winans. It was based on the Hayes Ten-Wheeler that was built from 1854. Like this, it had a long, narrow firebox for burning anthracite. Although the large cab was enclosed, the fireman had to walk out of it to access the firebox. The stack was a tall, parallel tube that was not located at the front of the smokebox, but far back directly in front of the cab.

While only 17 had been built of their predecessor, a total of 110 Davis Ten-Wheelers had been built between 1869 and 1873. They had come out of the Mount Claire Shops in Baltimore. Number 173, originally numbered 373, has survived until today and is one of only five remaining Camelbacks. It was donated to the Purdue University by the B&O in 1901 and 50 years later, it was again donated to the Museum of Transportation at St. Louis, Missouri. There it was on static display before the museum started to cosmetically restore it in 2017.

The first five locomotives delivered to the Baltimore & Ohio were not successful since they were too small. Nevertheless the B&O bought the patents of these locomotives and gave Phineas Davis the task to develop a heavier locomotive that could haul 150 tons with 15 mph. He was assisted by Ross Winans. The result was the “Atlantic” built in 1832 that used many design features of the “Tom Thumb”.

The “Atlantic” had a vertical boiler with a fan to increase the draft. The cylinders were also vertical and worked over levers onto a crank axle. These levers led to the nickname “Grasshopper” for this type of locomotive. The crank axle was connected to the two driving axles via spur and pinion, so the drivers turned twice as fast as the crank axle.

This locomotive is considered the first commercially successful one built in the USA. 20 more were built after the same plans. Although the original Grasshoppers had been scrapped, their second generation was used as shop switchers until the 1890s. One of these, No. 7 “Andrew Jackson” was rebuilt to look like the “Atlantic” and used for public demonstrations, for example at the “Fair of the Iron Horse” in 1927.