The Louisville & Nashville's 42 class M-1 2-8-4s are reported to have been the most expensive Berkshire type locomotives ever built, incorporating many of the latest improved features of steam locomotive design. No. 1967, in the upper builder's photo, was a member of the second group that came from Baldwin Locomotive Works in 1944, while No. 1970 in the lower photo represents the final group delivered by Lima Locomotive Works in 1949. Lima underbid Baldwin for that order, which was the second-to-last order for steam engines to be placed with Lima. There is some debate as to whether one of the L&N engines was, in fact, the last to be outshopped rather than Nickel Plate's 2-8-4 No. 779, generally considered to have been Lima's final steam product. The M-1s from Baldwin were delivered with web-spoke driver centers, but the Lima group came with Boxpok centers.

The M-1s weighed 447,200 pounds and delivered 65,290 pounds of tractive force. Their boiler pressure was 265 p.s.i., and they had 69-inch diameter drivers and 25x32-inch cylinders. Their evaporative heating surface totaled 4671 square feet, with 1908 square feet of superheating surface, and their grate area was just over 90 square feet. With dieselization of the L&N they began to be retired in 1950, and the last remaining members of the class dropped their fires and were cut up in 1956. Several locomotives in the first Baldwin group of M-1s, delivered in 1942, saw service on L&N passenger trains, especially Kentucky Derby specials — an almost unique case of Berkshire types hauling passengers during the steam era, although photos exist showing Southern Pacific's ex-Boston & Maine 2-8-4s with a passenger consist.