An understanding of steam locomotive specifications begins with the idea that locomotives are best judged by three inter-related qualities: strength, speed, and power. Strength, technically measured as tractive effort for locomotives (and torque for automobiles), is the force with which the locomotive can tug on its train. The available force is determined by the boiler pressure (higher pressure means more force), the piston bore and stroke (wider piston and longer stroke mean more force), and wheel diameter (smaller wheels, like smaller gears, mean more force). This can only translate into real strength, however, if the locomotive has good traction, otherwise it will just spin its wheels. Therefore the engine's weight on its drivers will limit its actual strength, particularly at low speed. At higher speeds, the locomotive's actual strength will be limited by the ability of the boiler to provide enough steam to the steam chest/cylinders. The engineer will gradually cut off steam to the steam chest cylinders using the reverser (aka Johnson bar) as the speed increases in a way analogous to shifting to higher gears in an automobile.
The speed a locomotive can achieve is limited by its stability and the strength and design of its moving parts. For a steam locomotive, stability is mainly determined by two factors: the number of front, unpowered axles, and the motion of the running gear. The front set of unpowered wheels, assembled in the so-called "front truck" or "pony truck," guide the locomotive through curves. With two wheels (a single axle) in front, the maximum safe speed is about 60mph. With a second axle, however, speeds over one hundred miles per hour can be achieved in regular service. However, the extra axle in the front truck takes weight off of the drivers, and limiting the locomotive's available tractive effort at lower speeds. The motion of the running gear—the system of pistons, main roads, and connecting rods that connect the drivers to the cylinders in the steam chest—generates lateral and vertical forces on the locomotive and the track that increase with speed. To minimize the magnitude of these forces, designers increase the diameter of the drivers and connect the running gear closer the driving wheels' axles, which translates into a shorter piston stroke. This gives the locomotive a higher gear ratio, increasing speed but reducing tractive effort. Designers can also use a narrower piston bore to achieve higher speed because the reduced force from the piston onto the running gear means the running gear can be built more lightly, further reducing the destabilizing forces.
By now, you've noticed that there are multiple trade-offs between power and speed, but there's one more factor—power. Power, often measured in horsepower, is the product of force (strength) and speed. This may be easiest to consider in terms of a constant-power system like your car. At low speed, your car can accelerate quickly, forcing you back into your seat more firmly than it can at high speed. A steam locomotive is not a constant-power system, but it is governed by the same principal—it can deliver a high force at low speed (pulling a heavy train slowly) or a low force at high speed (pulling a light train quickly). By the 1930's, however, railroads wanted locomotives that could pull heavier and heavier trains more and more quickly. The only way to do this was to increase the size and improve the shape of the firebox, the part of the locomotive typically just ahead of the cab that contains the fire that heats the boiler to generate steam. This could only be done by adding more unpowered wheels to the trailing truck to support the additional weight (designers couldn't just add more drivers because the wheels are tall enough to get in the way of the firebox). This takes more weight off of the driving wheels, which limits strength, so four-wheeled trailing trucks were typically used only when there were many driven axles (so that the ratio of driven to un-driven axles remained reasonably high) or when the locomotive was intended for light, fast passenger duty.
The Whyte wheel arrangement can be used to tell at a glance what the best use for a locomotive will be. The first number is the number of unpowered wheels in the front truck, the last number is the number of unpowered wheels in the trailing truck, and the middle number(s) is(are) the number of driving wheels. If the first number is a 4, the locomotive is capable of traveling at high speeds and is intended for passenger or fast freight service. If it's a two, it's either meant for freight or for passenger trains on steep grades where the speed will be limited. If the last number is a 4, the locomotive can generate a lot of power to pull a medium or heavy load quickly. The ratio of the number of drivers to the number of unpowered wheels will tell you whether the locomotive is meant for freight service, passenger service, or dual duty. Remember, you need a large proportion of the locomotive's weight on its driven axles if it is to pull hard enough to start a heavy freight train or lug it up a steep grade. Generally, if less than 50% of the wheels are driven, the locomotive will be used for passenger service, and if roughly 67% or more of the wheels are driven, the locomotive will be used for freight or for switching duty. The SP&S 700, like all 4-8-4 "Northern" locomotives, are right at 50%, and it excelled at both fast freight and passenger service, but it was intended primarily for express passenger service.
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