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.