In fact, the engine is a so-called, “big-boy,” designed in 1941 for the Unon Pacific Railroad in the US. These were the biggest steam locos ever produced and used.
It’s worth considering how and why things got so big…
A steam loco is an arrangement of parts: fire-box, boiler, and driving wheels. These elements have to be put together to optimise performance within a context defined by the scale of operation (distance), geography (terrain), and all of the existing infrastructure of the railway (loading guage, curves etc). This last is not just about the distance between the rails, it’s also about the size of existing tunnels, and bridges etc.
You can see that, in this context, every engine is a kind of heroic compromise…
In the US context of these enormous engines, the task was to design an engine with the power to pull huge loads up steep gradients, and to keep going across long distances.The engine needed both power and stamina.
By the 1940s, this was the last throw of the dice for steam traction faced with new developments in diesel and electric locos.
A steam loco eats coal and water…so, fitting a bigger tender increases the range of the engine and improves performance. You can reduce the number of stops for taking on water and extra coal. Accordingly, the tender on these engines was massive; it had seven axels and shifted coal to the firebox using an automated conveyor.
The firebox was the size of a room. It measured nearly six by two and a half meters, and heated the boiler to provide steam to two sets of driving wheels.
The arrangement of wheels on a loco is recorded using Whyte notation. In this case, the engine is identified as a 4-8-8-4.
You can probably guess that something this big is great on the straight…but also has to get around the existing curves. To allow these continental scaled engines to take the existing curves of the track in their stride, they are articulated according to the Mallet design of engine.
The Big Boy fleet of twenty five locomotives were used primarily in the Wyoming Division to haul freight over the Wasatch mountains between Green River, Wyoming and Ogden, Utah, in the US. They were the only locomotives to use a 4-8-8-4 wheel arrangement consisting of a four-wheel leading truck for stability entering curves, two sets of eight driving wheels and a four-wheel trailing truck to support the large firebox.
Led by mechanic Otto Jabelmann, the Union Pacific Railroad’s design team worked with the American Locomotive Company. The team found that Union Pacific’s goals could be achieved by enlarging the firebox, lengthening the boiler, adding four driving wheels and reducing the size of the driving wheels from 69 to 68 in (1,753 to 1,727 mm) on a new engine. That’s how things got to be the size they got.