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 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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