Extract from "Model Railroader 1985" Article by MARC HOROVITZ Volume 25 No10 
                                 STEAM ENGINE THEORY
     In its most basic form, a steam engine is little more than a piston (much like an automobile
  Piston) that is moved in a cylinder by the pressure of steam. The cylinder may be single acting
 meaning that the piston is shoved in one direction only,relying on the kinetic energy stored in
 a flywheel to return it to its original position, or it may be double-acting, meaning that the
 piston is pushed back and forth by alternately admitting steam into opposite ends of the 
 cylinder.
    The device that admits and exhausts steam to and from the cylinder is called the valve.There
 are several different types of valves; they are fully covered in technical bocks on steam and 
 need not be extensively explored here. Basically, the valve moves back and forth, covering and
 uncovering holes or ports that admitsteam to the cylinders, and allowing the exhaust to escape 
 so that the piston may return. Obviously this must be done with some precision, so the valve 
 must be accurately controlled.
    The devices that control the valve are called valve gear. The valve gear can be as simple as
 an eccentric on the driven axle, or as complex as one of the full size locomotive valve gears, 
 such as Stephensen's Link or Walschaerts. All valve gears perform the same function in different
 ways, admitting and releasing steam from the cylinders with a greater or lesser degree of 
 efficiency.
    There is, however, an extremely simple cylinder that requires no valve at all. See the 
 accompanying drawing. This is the oscillating cylinder, which saw limited use on fun-size 
 locomotives. The oscillator may be single- or double-acting, and this is how it works. The 
 cylinder is fiat along one side, and it pivots on a trunnion placed halfway down its length. 
 The flat face of the cylinder bears against a steam block. The trunnion penetrates the block 
 and is secured on the back of it, thus  binding the cylinder to the block, but al lowing it 
 to rotate on the trunnion.
     A hole is made in the flat face of the  cylinder at each end to admit and exhaust  steam. 
 The piston rod is attached directly  to the wheel. As the wheel turns, the piston and cylinder 
 are forced to oscillate up  and down. At the top and bettom of the  wheel's revolution, the 
 cylinder is at two  extreme positions. There are four holes  made in the steam block, two each
 to  mate with the holes in the cylinder at  each of its extreme positions. These are  the
 admission and exhaust ports. All you  have to do is apply steam to either of the  ports to make 
 the engine run. ff the engine is going the wrong way, simply reverse the admission and exhaust 
 and you  have reversed the engine.
     To make the steam for the cylinders, a  boiler is required. Obviously, a locomotive boiler
 must not only be functional,  but must also be designed for the specific engine that it is 
 mounted on. There  are dozens of boiler designs and variations, but they all fall into two 
 basic categories: externally fired and internally  fired. An externally fired boiler (also
 referred to as a pot boiler) is just that. The fire is entirely outside the boiler. An 
 intornally fired boiler has tubes running through it that carry hot gases on their way from the
 fire to-.the smokebox andstack. These tubes increase the surface area (or heating area) of the
 boiler and make it more efficient. However, the tubes cut down the water capacity of the 
 boiler, which is an important consideration in small scale steam.
     Another way to increase both surface  area and water capacity is to extend  tubes full of
 water down from the boiler  into the fire. This is fine, but it de creases the fire space. Thus,
 a good  boiler is a delicate balance of many factors. Most of the commercially available 
 locomotives have well-designed boilers.
     The steam is transported from-the  boiler to the cylinders through a steam pipe or line. 
 The amount of steam is controlled by a sensitive valve, commonly known as the throttle.
 Depending upon the model's sophistication, the boiler may have fittings on it to help run the
 engine or provide information about the state of the boiler to the engineer. 
    Many of the other fittings are miniature versions of controls found on fullsize locomotives.
 These fittings may or may not be found on a specific model locomotive. The blower valve controls
 a jet of steam released up the stack to create a partial vacuum that pulls the fire and heat 
 through an internally fired boiler. A reversing lever may be used if the locomotive is 
 sophisticated enough to have working valve gear. A pressure  gauge tells the engineer if steam
 is up or down and can give an indication of the state of the fire, which is not. always visible.
 A water glass helps the engineer keep track of the amount of water in the boiler. Some engines
 have a blowdown valve which allows the engineer to safely discharge the remaining steam from the
 boiler at the end of the run. 
    Every steam locomotive must also have at least one functional safety valve. This is an 
 automatic valve set atop the boiler that releases excess steam pressure. A boiler is normally 
 tested to at least twice its working pressure, so this valve maintains a large margin of safety.
 The safety valve is something that should never be tampered with, unless you are very know-
 ledgeable on steam matters and you know precisely what you are doing.

                        OPERATION
    One of the joys of small scale steam is getting to know your locomotive. Every engine has its
 own personality, and it takes some practice to get the best performance out of any given engine.
 This quality has the tendency to bring the engines to life, to make them personal machines.
    A live steam locomotive is not as difficult to operate as one might imagine, particularly in
 these smaller sizes. On a fullsize locomotive, the operator must watch the water level, the 
 state of the fire, the boiler pressure, and several other things. Many small engines are
 designed so that the boiler and fuel reservoirs may be filled and left alone until the end of 
 the run. The fire is usually designed to go out before the boiler is empty, to prevent damage
 to the boiler. Thus, the model engineer's greatest concern is the speed of the train, which is
 easily regulated by adjusting the throttle.
    Let's pretend you're the engineer for a day's run with a live steam locomotive. A typical 
 operating session begins at the steaming bay (a short length of track used only for raising 
 steam), or you may raise steam on a bench or on the main line. First, the engine must be
 carefully examined to make sure that everything is in good working order. Try moving all of
 the controls to ensure that they are not frozen. Lift the safety valve manually to make sure 
 it isn't stuck. Next, oil all of the moving parts: crank pins, axles, cross heads, etc. This 
 oiling must be done before every run.  After you finish the inspection, fill the boiler with
 distilled (Treated)water. This may be done through a filler plug, or you may choose to pump the
 water in through a pump built into the engine or at trackside. You have to carefully watch the 
 water level coming up in the water glass to keep from overfilling the boiler. When the boiler 
 is filled to the proper level, add steam oil to the engine's lubricator. Steam oil is a very
 viscous special oil formulated for use in steam cylinders as regular machine oil tends to 
 disappear immediately under the conditions found inside the cylinder. Fill the alcohol tank and 
 then adjust the drip feed to the burner. You're now ready to light the fire.
    Extract from Model Railroader 1985 Article by MARC HOROVITZ Volume 25 No10 
                 This Section added by R.Stirling.
    My limited experience with Lve Steam Narrow Gauge 3.6" Locomotives ie W901 at Peterborough 
 South Australia , shows a slight difference from Model Steam per ie:
 Lighting the Boiler as we do it involves the pre check as above, then the Fire is Lighted by 
 adding a few old oily rags and Kindling Wood, this then is set alight with a piece of oil soaked 
 rag on the  end of a Special Rodd , the fire is then fed with pree cut waste wood or old 
 sleepers which have been  broken up so as to fit through the Coal Feed opening in the rear of
 the Fire Box,after two or three hours depending on the size of the boiler the Pressure will
 rise to between 150 / 300 PSI , the Steam Pressure  Valve will release when maximum pressure 
 is reached. The Engine is moved into motion by opening the main steam valve, which involves
 the driver moving a large lever which in turn opens the main power Valve located atop the
 boiler in "The Dome" the steam then goes to the Cylendar Valves which control the Cylendars
 and Driving Rodds. 
                              "Note This IS NOT GOSPEL "