Steam loco operation

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The first diesel-electric appeared on the Baltimore & Ohio (B&O) rails in 1935. Some roads were quick to completely dieselize, but for the most part, WWII delayed the inevitable until what most call the "transition era" of the 1950's. By the early 1960's most steam had been retired, though a few shortlines continued to run some steam through the 1980's.

If you do your homework, you'll find various histories depending on individual railroads. But for the most part, 1950-1960 spelled the end of steam.