The various steamers that where built over the years vary substantially, as virtually no two were exactly identical, even of the same design type! There were, however, roughly three basic types; non-articulated (meaning a rigid frame), duplex (it divides the driving force on its wheels by having two pair of cylinders but is also housed on a rigid frame), and articulated (which is a rigid mounted rear frame but pivoting front frame to negotiate curves). Perhaps the best known and recognized of the early locomotives types was the very successful American Type 4-4-0 design, built during the mid 19th century which found widespread use across the country. From that point on steam designs continued to grow in size and weight and reached their technological height during the 1940s, at which time the industry began switching to diesel-electrics and steam began to fade away until it was all but completely phased out by the early 1960s.
Speaking of locomotive designs you may be wondering, “What the heck do those numbers and dashes mean?” The “scientific” or technical term for these numbers and dashes is the “Whyte notation” (developed by Frederick Whyte), which classifies steam locomotives by wheel arrangement. For instance, how the system works is by counting the number of leading wheels (those found on the front of the locomotive or pilot), then the number of driving wheels (those found directly under the boiler), and finally the number of trailing wheels (those found under the locomotive's cab), all separated by dashes.
So, if you happen to see a steamer and are curious as to how it is classified, simply count the wheel groupings from the front (the pilot) to the back (the cab), remembering that if you are looking directly at the side of a steam locomotive to double its wheel count when classifying (for instance, if you see three main driving wheels on one side there are also three on the other side for a total of six in your classification). For instance, with the common American design, it has four front wheels of two axles, four main driving wheels (or just drivers), and no trailing wheels/axles, and thus is classified as a 4-4-0 design.
Having said this you may also be wondering what the purpose is behind those trailing and pilot wheels/axles to either end of the main drivers. Well, the front pilot wheels/axle(s) acts as a guide to better help the locomotive negotiate its way into a curve without jumping the track, which was especially helpful as locomotives grew larger and heavier. The rear or trailing wheels/axle(s) served mainly in a supporting role as locomotives grew larger to bear the weight of the heavier cabs and fireboxes. These front and rear wheels/axle(s) also served a secondary function in displacing the weight of the locomotive more evenly over the rails, which produced less wear on them.
Perhaps more than any other piece of railroad equipment, past or present, the steam locomotive is the most universally recognized piece of railroading equipment. A good example of this is how children typically perceive the main line freight trains of today, calling them "choo-choos" even though the train is being pulled by diesel-electric locomotives, and steam has been gone for nearly five decades now [from hauling freight]. Of course, there are many good reasons why steam is so widely recognized as relating to railroads. First is by the far the most obvious that, again, steam pulled the trains for the first 130+ years of the industry’s existence and during that time the American public traveled by train in much great numbers during than they do today.
Perhaps the most compelling reason why steamers are so fascinating is that few other pieces of machinery can so visually show just how hard it is at work (by being able to see the rods and drivers in motion). If you were lucky enough to be trackside during steam’s golden years, from the 1900s to the 1940s, the experience was truly awesome; such as witnessing the Milwaukee Road's famous Hiawatha passenger train zipping by at over 100 mph through the Midwest with a 4-4-2 Atlantic or Hudson 4-6-4 on the point, or watching a thunderous display of power as a Baltimore & Ohio 2-6-6-4 EM-1 lugged a coal train over the railroad’s steep main line in West Virginia and Maryland. In recent years some of that magic has been rekindled thanks to restorations of several large steam locomotives including Norfolk & Western J Class 4-8-4 #611, Union Pacific 4-8-8-4 "Big Boy" #4014, and Chesapeake & Ohio 2-6-6-2 #1309. For more information about steam locomotives please click here to visit Wes Barris' authoritative site on the subject.
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