Lately I’ve been noticing how on locomotive the number, size and position of the wheels can vary a lot. This was also true for the old-style ones, like the steam trains. What are the engineering principles that drive this decision?
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For historical steam engines, the first thing considered was weight. Nearly all steam locomotives of the 1700s – early 1900s were powered by water boilers, mainly water tube boilers. Most often made out of thick wrought iron, they were HEAVY. VERY HEAVY. Especially when they were filled with water. While not a hard-a-fast rule, generally, steam locomotives weigh more than diesel or diesel-electric locomotives of the same size. You needed large, robust wheels to support all that weight. Generally, though, fewer axles (and thus larger wheels) were preferable because there was fewer total number of moving parts and thus lower maintenance costs.
Secondly, the method of propulsion on steam locomotives is a steam piston. The large pistons are connected to the long rods you see attached to the sides of those wheels. You’ll notice, the rods are not connected to an axle, but the wheels themselves, a certain length from the center of the wheel. The back-and-forth action from the steam piston rotates the wheel(s). The larger the wheels, the faster a piston could technically rotate them, but there would be a larger amount of inertia to initially overcome. And on many trains, not all running wheels are even connected to the rod.
And this is where things delve into the idea of “what really worked best?” There isn’t a clear answer, as you’ll find a variety of wheel combinations on steam locomotives. It all boiled down to a balance of speed, stability, articulation, and train size (and the aforementioned maintenance). Although I don’t like linking to Wikipedia, this [page](https://en.wikipedia.org/wiki/Whyte_notation) has a good overview of many of the historical combinations.
Nowadays wheel arrangement is more about efficiency, both in cost and operation. That brings you to the front-and-back 4 and 6-axle diesel and diesel-electric locomotives, mostly used for pulling massive loads, but not necessarily very fast. This is for your typical freight train. It’s a good balance of supporting the locomotive itself and the load it has to pull, while also providing sufficient traction on the rails without sacrificing efficiency. (You could add plenty more wheels to the train to create more points of tractive effort on the track, but that would reduce efficiency since you now have more friction points, and also increase manufacturing costs.) There are probably many different axle examples, but 4 and 6 is probably the most common in North America. But there’s no reason to over-engineer the locomotives because you can always just add another to pull more!
For steam, there are a lot of competing factors.
With few exceptions, steam locomotives are direct drive, they don’t have a gearbox with low gears for extra pulling power (hills and heavy loads) and high gears for efficient fast cruising. That means driving wheel size is one of the only ways you decide that. So, anything that wants to go fast wants larger wheels (for “higher gear”), and pulling heavy loads wants smaller wheels. This is why passenger locomotives have the largest wheels and low-speed freight locomotives the smallest, with mixed-traffic in between.
As time went on and technology advanced, pistons could work faster, allowing “higher RPM” more or less, which allowed for passenger locomotives with smaller (and more) wheels to still go fast, but now pull heavier loads. This is why 1850s fast passenger locomotives often have two *enormous* driving wheels, but 1930s fast locos have six or eight, and are pulling longer trains with heavier (steel) cars.
The more weight of the locomotive that is on the driving wheels, the more it can pull. But at the same time, a leading unpowered axle or two aids stability and turning at high speed. Trailing axles also add stability, but another purpose is to allow a larger firebox for more power at high speed. See, driving wheels are large enough that they restrict the depth and width of a firebox, while trailing axles give more room. This is why passenger locomotives have more unpowered wheels.
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