The best known example of the cab-forward design in the United States, the Southern Pacific Cab-Forward (also known as “Cab-in-fronts”) placed the cab at the front by the simple expedient of turning the entire locomotive, minus the tender, by 180 degrees, an arrangement made possible by burning fuel oil instead of coal.
The cab forward design was widely used by the Southern Pacific Railroad, which developed it to deal with the peculiar problems of its routes. The 39 long tunnels and nearly 40 miles (64 km) of snow sheds of the Sierra Nevada Mountains could funnel dangerous exhaust fumes back into the crew compartment of a conventional locomotive. After a number of crews nearly asphyxiated, someone had the idea of running his locomotive in reverse. This meant that the tender was leading the train, which introduced new problems. The tender blocked the view ahead and put crewmen on the wrong sides of the cab for seeing signals. The tenders were not designed to be pushed at the lead of the train, which limited speeds. Southern Pacific commissioned Baldwin Locomotive Works to build a prototype cab-forward locomotive, then ordered more before the prototype had even arrived.
All of the cab-forwards were oil-burning locomotives, which meant there was little trouble involved putting the tender at what would normally be the front of the locomotive. The oil and water tanks were pressurized so that both would flow normally even on uphill grades. Visibility from the cab was superb, such that one crewman could easily survey both sides of the track.
Big Boy could generate a maximum of 6,290 drawbar horsepower. The Big Boys were the only locomotives to have the 4-8-8-4 wheel arrangement in the Whyte notation, combining two sets of eight driving wheels with both a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox.
The Big Boys rendered important service in the Second World War, especially since they proved so easy to fire that even a novice could do a fair job. Since many new men who were unsuited to combat service or exempted were hired by the railroads to replace crewmen who had gone to war, this proved advantageous. During the war, after German agents filed reports that the Americans had giant steam engines that were moving huge trains full of vital war material over steep mountain grades at high speed, their reports were dismissed as “impossible”. Their performance in moving a huge volume of war material throughout WWII was repeatedly cited and the Big Boys are generally acclaimed as having made a huge contribution to the war effort