[Jeff Higgins]

That is just so freakin' awesome. I might just have to go and see that thing run.

My younger brother and I were pretty avid model railroaders back when we were young. We became big fans of the Norfolk and Western Y6B, a 2-8-8-2 configuration. It had more tractive effort, but less horsepower than the Big Boy. It was built for hauling their long coal trains so, as such, had smaller diameter drivers. It was our main motive power, with three units serving our make believe iron ore railroad. I believe a company called AHM or something made them, at a price a couple of kids with paper routes could afford. Big Boys and Challengers were only available as expensive brass models, so we couldn't run those.

For those unfamiliar with steam locomotives, driver diameter is how these things were "geared" for their different uses. High speed passenger locomotives could have as large as 80" in diameter drivers. Freight locomotives could have drivers as small as 53" diameter, or less. Slower speed locomotives also had fewer wheels on the pilot truck, the non-driven wheels under the smokebox. The Y6B, for example, only had two wheels on the pilot truck, as denoted by the first "2" in the 2-8-8-2 nomenclature. Intended for higher speeds, the Big Boy had four, as denoted by the first "4" in its "4-8-8-4" designation. Oh, and the last number indicates how many wheels are under the firebox, so the Y6B had two, the Big Boy four (these are, like the pilot truck wheels, not driven). This is a rough indication of horsepower potential on a steam locomotive - the size of the fire box. Bigger ones heat more water faster, generating more horsepower.

The Y6B ran on 58" drivers, so was intended more for heavy, slow freight. The Big Boy ran on 68" drivers, a compromise meant to haul fright at speed, or occasionally haul passenger trains. Their pilot truck and trailing truck configurations support these differences in use as well.

It's pretty easy to guess the intended use of a steam locomotive just by looking at its wheel layout and size. Engines confined to the yards, called "switchers", typically have no pilot nor trailing trucks, and quite small drivers. 0-6-0, 0-8-0, are typical wheel layouts. The ubiquitous freight engine was the 2-8-2 "Mikado", typically with somewhere around 63" drivers. A common passenger engine was the "Pacific", a 4-6-2 configuration with 74" to 80" drivers.

Articulated engines, with two sets of drivers like the Big Boy, came in two configurations, power-wise. The original articulated engines were "compound" engines, wherein the exhaust steam from the smaller, high pressure rear cylinders (that were fed directly from the boiler) was fed to the larger diameter, lower pressure front cylinders. These are easily discerned by their larger front cylinders. These were known as "Mallets", for whatever reason.

The Big Boy and other modern articulated engines were "simple", rather than "compound" engines. Both sets of cylinders got steam directly from the boiler. These are easy to pick out because all four cylinders are the same size. These are sometimes called "Mallets" as well, but that is technically incorrect. "Mallets" are compound engines, if we are picking nits.

Anyway, I think this is all pretty darn cool stuff. I understand the economics of it, but darn it, our railroads really lost something when they gave up steam. I cannot imagine what it cost to get this Big Boy up and running again, or what it will cost to keep it that way, but I sure am glad they did it. Just so awesome...