Starting is a big deal for a steam locomotive. Unlike the electric motors in diesel-electric locomotives, which deliver maximum power at start-up, steam locomotives are wimpy at startup and gain power with speed. This is the result of a fixed ratio between the pistons and the wheels: the pistons move too slowly at start-up to use the available steam power (you can’t “down shift”). There are four ways to help the locomotive start: increase the coefficient of friction, decrease the weight being towed, maximize the leverage of the rods that crank the wheels, and temporarily increase the number of driving wheels.
To move a train, the locomotive’s drivers must push back against the rails without slipping. The “coefficient of friction” is the percentage of the weight that can be converted into horizontal force before the wheel slips, about 25% for steel wheels on steel rails. One hundred tons on the driving wheels translates into twenty-five tons of pulling force.
So the first step to beef up the starting force is to increase friction by sanding the rails. For this purpose, steam locomotives carry sand in one or two large domes atop the boiler, kept dry by boiler heat. Tubes from the sand domes, each with a valve, curve around the side of the boiler and then around the perimeter of each driver, terminating just above the rail.
The second trick, if you are hauling uncomplaining freight, is to reduce the load you are pulling. This is possible because in American trains lack the “buffers” used in European trains to keep the couplings taut, allowiwng some slack in the couplings between cars. So you set the brakes at the back of the train, and back down until the couplings are compressed together. Then you take off like a bat out of hell, pulling first one car, then two, gathering more and more cars, until the last car, always the unfortunate caboose, became part of the chain only when the train was already moving at 5 or 10 mph. Serious injury was the fate of conductors caught unaware, and broken “drawbars” that connect the coupling to the car were common. The sound of a starting freight train was memorable, as the clank of slack couplings coming together ran like a zipper down the train, a kind of rolling thunder.
With a passenger train this approach can’t be used. It was a matter of pride for the engineer not to “spill the soup” when starting. Instead, you stretch out each coupling so all the cars move at once, with no start-up jerk. You are stuck with the whole load.
The third move is back up a bit until the rods that turn the wheels are at the angle at which they exert the most leverage.
The fourth move was available on a few locomotives, including the SP’s 4-8-4’s. These were equipped with “boosters”, a compact steam engine set between the wheels of the trailing truck, that added the weight on that truck to contribute to the tractive force at low speeds. They added almost 25% tractive force while emitting furious sideways snorts of steam, in detailed counterpoint to the much larger main drivers. It made a glorious show for the enjoyment of a track-side teenager.
So the engineer adjusts the position of the wheels, sands the rails, turns on the booster, applies steam very carefully, and hopes for the best. A youth at track-side hopes for the worst: the drivers lose their grip, and the locomotive rapidly chuffs and clanks in place, like a trained horse.
The gradually accelerating tempo of the exhaust blasts has been imitated many times in music. Villa-Lobos’ The Little Train of the Caipira is a delightful piece that vividly captures the sound and motion of a train ride (listen and watch the wonderful graphics at https://www.youtube.com/watch?v=1rRFDnTEu6g ). Prokofiev intentionally or unintentionally captures the essence of a starting locomotive in the second movement of his great 5th Symphony. You can hear it at https://www.youtube.com/watch?v=-e0c4GRx4So : the start-up begins at 5:06, but I encourage you to listen to the whole movement – it is a thrilling performance (learn more about this remarkable youth orchestra at the Wikipedia entry for “El Sistema”). Train enthusiast Arthur Honegger’s 1923 composition Pacific 2-3-1 is best heard at https://www.youtube.com/watch?v=T9u7_WAkAPw. I also found a wonderful 10-minute 1949 film that captures the excitement of the steam locomotive, using Honegger’s score. The clearest video on YouTube is at https://www.youtube.com/watch?v=rKRCJhLU7rs (it is the better for being without sound).