The K-8 was the last class of Consolidations ordered by the Boston & Maine. Although originally designed with a saturated boiler, it weighed more than 200,000 pounds, compared to less than 180,000 pounds for all previous classes of this wheel arrangement. The drivers had a diameter of 61 inches and the cylinders measured 22 by 30 inches. The first batch of 40 locomotives, designated K-8a, was built by Baldwin in 1911. Baldwin built 50 more superheated locomotives in 1913 which were also designated K-8a. ALCO-Schenectady delivered 20 saturated K-8b in the same year and ALCO-Brooks added 25 superheated K-8c in 1916.

The superheated locomotives had a reduced boiler pressure of 180 instead of 200 psi and a cylinder diameter that was two inches larger. The first 40 Baldwin locomotives and the 20 Schenectady locomotives were also superheated in 1916 and designated K-8d. From 1924, they were equipped with thermic syphons. Starting in the late twenties, they received Worthington feed water heaters. The class was retired between 1937 and 1954. Two were sold to the Bangor & Aroostook in 1946.