Here are some objects you may see in This Old Choo Choo.
Boiler, fueled by the fireman, raises steam to drive the pistons. The fire's heat travels through a bundle of tubes into the smoke box.
These hot tubes boil the water in the drum. Steam bubbles rise to the top, collecting at a high spot called the steam dome.
A pipe here collects mostly-dry steam and carries it to the cylinders. The boiler has one more important thing to do with the steam! Leftover steam from the cylinders puffs out the blast pipe, which points straight up the smoke stack. This
blast of leftover steam pulls fresh air, heat, and smoke through the firebox, tubes, and smoke stack. This helps the engine run harder and causes the familiar puffing noise.The steam pressure depends on how hard the boiler is fired, and how the steam is being used in the cylinders. Maximum pressure for an engine of this type was 150 pounds per square inch.
The engineer makes sure that the boiler has enough water. Fresh water must be pumped in to replace what has boiled away. If the water level in the boiler gets too low, the metal surrounding the firebox will get hot and weak, resulting in an explosion with certainly fatal results. On the other hand, overfilling the boiler might get water into the cylinders, causing a less drastic but nevertheless career limiting explosion. Frequent checking of the sight glass and try cocks is advisable!
Running Gear follows the 4-4-0 pattern favored for fifty years in the early days of American railroading. The four leading wheels provided stability and flexibility
to follow the poor track conditions common in North America. The large drive wheels straddle and support the firebox. A slider and crank mechanism converts forceful stroking of the cylinder into rotation at the wheels.The diameter of the large drive wheels, and the location of the crank pin on the wheels, set the gear ratio of this mechanism. This Old Choo Choo is a general purpose locomotive with an average gear ratio. Smaller wheels with a crank pin further from the axle might be found on a slow, high traction locomotive used for heavy freight.
One might notice that, when the piston is at the end of its stroke, the position of the crank does not allow it to turn the wheel one way or the other. But, when the action on the near side is in this difficult position, the action on the far side is safely in mid stroke, so one way or another the wheels will receive effective power.
Cylinders (one on each side of the engine) convert steam pressure and flow into forceful stroking of the piston rod. These are dual stroke cylinders, meaning that
they provide power in each direction of the piston stroke.
The slide valve in the chamber above the piston regulates the action. When this valve strokes, it admits fresh steam to one side of the piston, and exhausts leftover steam from the other side. The
fresh, high pressure steam pushes the piston to the other end of the cylinder, where the valve strokes again to continue the cycle.The cylinder shown above is in mid stroke, piston moving to the right. The valve has fully opened the right end of the cylinder to the exhaust. The left side, moments earlier, was open to the steam inlet, but is now closed to allow its breath of steam to expand efficiently.
When running under an easy load, the valve opens up for just a moment to let in a short breath of the fresh steam. This steam expands slowly and efficiently through the full stroke of the piston. To get more power, the engineer adjusts the valve motion to stay open during the entire piston stroke. This gets the most power, but inefficiently since the steam does not expand much before it goes out the exhaust. An engineer running this hard without good reason was liable to earn resentment from the weary fireman.
For a slightly different look at the valve and piston action, try this movie.
