Built between 1914 and 1919, the Pennsylvania Railroad's fleet of L1s Mikados hauled freight through two world wars and served until the end of steam in 1957. Designed by the railroad's own mechanical engineers in tandem with the passenger-service K4s Pacifics, the Mikes replaced 2-8-0 Consolidations as Pennsy's main line freight power.
The Mikado followed the Pennsy's practice of testing a locomotive design thoroughly and then building a standardized engine in large numbers. The L1s class - 574 strong - and the 425 class K4s Pacifics shared an identical boiler and many other common parts, giving shop crews nearly 1000 locos that were largely the same. While the "s" in L1s stood for superheater, the Mikados had few other modern appliances when built. The Walschaert valve gear was reversed by a heavy screw mechanism and the large firebox was hand shoveled.
By the early 1930's the Interstate Commerce Commission had mandated the use of power reverse gear. The addition of this mechanism to the engineer's side of the engine pushed the air tank to the front of the boiler, giving the L1s the characteristic face it wore in later years. Many, but not all, Mikados eventually received coal stokers to feed the firebox, adding about 1000 horses to their original potential of about 2700 horsepower. Over level or moderately hilly terrain, an L1s could handle a 70 - 80 car freight train. In one instance, a Mike hauled two GG1's with their two passenger consists through a 10-mile stretch after the overhead wire became disabled.
Just as the Mikados displaced lesser 2-8-0's, however, they were themselves bumped from mainline service by the advent of Decapods and other larger power. The Mikado fleet spent many of its later years going in and out of mothballs as the Pennsy's freight business ebbed and flowed. When every piece of available power was needed during World War II, Mikados that were still hand-fired had two firemen shoveling coal. Many of the dependable Mikes worked in branch line, helper, and even railfan service until the end of steam on the Pennsylvania Railroad.