The conventionally-oriented steam locomotive, with its front-facing boiler, and rearward cab, was suitable for nearly every application in railroading, and was generally the universal preferred standard; in fact, on the eve of dieselization, some roads ordered diesels long-hood forward, as locomotive crews enjoyed the distance protection offered by a steam locomotive’s boiler in a collision.
However, the design had its drawbacks under certain conditions. For one, on routes with numerous tunnels in mountain territory, the locomotive’s exhaust would fill the tunnel, especially when working upgrade, and make breathing difficult for the locomotive crew. Some railroads fitted oxygen breathing tubes fed by a regulating valve off of the locomotive’s air reservoir into the cab, to circumvent this issue; the Southern Pacific took this one step further, and developed a full breathing hood to be worn by engineers when running over the Sierras, where the combination of long tunnels, and frequent snowsheds made breathing hazardous on the constant upgrade runs:
However, sometimes the issue was visibility; as locomotives grew larger, or other variables, such as extreme curvature, presented themselves, the length of the boiler became a visual obstruction to the engineer, and fireman. In other cases, such as locomotives bearing an unusual mechanical characteristic, the cab-rearward design was simply viewed as unsuitable for proper balance, and operation. Thusly, for roads or mechanical engineers with such issues, the solution was to, quite literally, flip the locomotive around.
Running locomotives in a cab-forward arrangement, or with the engineer in a forward position, was nothing new to railroading by the 20th century. In fact, Matthias Forney’s locomotive patent of the late 1860s, which combined a small tender tank with the locomotive, was intended for fuel bunker-first, or bidirectional running; the idea being that the trailing truck supporting the weight of the bunker would act as the lead truck for more stable running of his compact design. Many of the earlier Forneys were built with this principle in mind, though later locomotives of the type moved toward conventional cab-rearward operation. The first 2 foot gauge locomotive built in the United States, the 1877 Hinkley-built Ariel for Maine’s Billerica & Bedford Railroad, illustrates the cab forward variant of a Forney, with the pilot and headlamp on the bunker end of the locomotive:
However, the first true cab forward in the United States, with a separate tender, was the North Pacific Coast Railroad’s #21. Officially named the “Thomas Stetson, and unofficially named “The Freak” by crews, #21 was an unsuccessful experiment rolled out of the railroad’s shops in 1901; the unholy marriage of a watertube marine boiler, and the wrecked frame of an old 4-4-0. For a tender, the 4-4-0’s old tender tank was scrapped, and vertical tanks for water, and fuel oil, were installed. Due to the watertube boiler, which didn’t respond well to the inherent large fluctuations in steam demand experienced by a locomotive, #21 was constantly wanting for steam. Additionally, the orientation of the atomizer (burner) in the firebox wore-out the water tubes prematurely, and the locomotive was often in the shop. To make matters worse, due to poor weight distribution, the locomotive was extremely slippery, and had a hard time getting traction on grades. She was unceremoniously scrapped after only four years, in 1905.
Most notable of all the American cab forwards, of course, were those ordered by the Southern Pacific, starting in 1909 with 15 Baldwin-built MC-2 class 2-8-8-2’s, built to lift freight trains over Donner Pass, and get through the Sierras without asphyxiating crews. The locomotives were oil-fired, as were all of those built later, and highly successful:
Two years later, in 1911, 12 MM-2 class 2-6-6-2’s followed, and were used in passenger service. Later, a four wheel pilot truck was added to increase their stability, and the MM-2’s became the AM-2 4-6-6-2’s , and lasted in passenger service until 1929. Note the whaleback tender, which was starting to gain prevalence for its increased visibility when backing, or looking over the train.
The Southern Pacific Dynasty of Cab-Forwards eventually expanded from the MC class (which went up to the MC-6’s) into the AC class, which stretched from the AC-1’s, to the 1943-44 built AC-12’s (skipping the AC-9s, which were cab rearward, for mostly desert running).
Today, only one of the SP Cab Forwards exist, AC-12 #4294, at the California State Railroad Museum in Sacremento; a testament to Necessity, the mother of Invention.