Steam locomotives operate by converting heat energy into mechanical energy.  This requires a heat source, a boiler to generate steam and cylinders converting the thermal energy contained in steam to mechanical energy, which can be used to perform work.  Steam is technically defined as being elastic aeriform fluid in which water is converted when heated to its boiling point.  Steam is invisible.  The visible white cloud commonly called steam consist of small particles of water into which steam condenses upon contact with cool air.

Steam exists in three different states.  When it is in contact with the water from which it was generated, but does not physically hold water in suspension, it is known as saturated steam.  Its temperature will always correspond to its pressure.  If it physically holds water in suspension (i.e. contains water droplets), it is known as wet steam.  If it is separated from its generation source and further heated, its pressure remains constant, yet its temperature is increased, then it is called superheated steam.   In the superheated state, steam contains more energy than in the SATURATED state and is capable of doing more useful work..

Modern steam locomotives were designed to meet requirements specified by the railroads, which included speed, load and fuel requisites.  Factors such as a locomotive's weight and size were also of importance since existing track, tunnels and bridges had strength and dimensional limitations, which could not always be changed to accommodate a new design.

During the 19th century, a systematic development process evolved which is partially applicable to modern miniature locomotive design.   Selecting the proper wheel arrangement to meet operational requirements is one of the most important decisions a locomotive's designer must make.  The term "tractive effort" is the name given to the force which is developed between the drive wheels and the rail, and is directed along the upper surface of the rail.  It is a function of the cylinder steam pressure, piston diameter, piston stroke and drive wheel diameter.

The term "factor of adhesion" is the ration of the tractive effort and the weight of the drive wheel.  It ranges from 0.20 to 0.33.  For a given weight and boiler pressure, tractive effort can be increased up to some maximum value, as limited by the factor of adhesion, by increasing piston diameter, stroke or reducing wheel diameter.  Beyond the maximum value of tractive effort, additional weight must be put on the drivers.  This can be done by building a heavier locomotive, however another limitation comes into play; the strength and size limits of the existing track and its associated structure.

At high speeds, a locomotive's tractive effort is limited by its boiler's ability to generate steam.  Two limiting design conditions have therefore emerged: cylinder and wheel dimensions for low speeds and steam generation for high speeds.  These limits make up the "design envelope" and a successful locomotive is one in which he limits have been judiciously balanced so as to obtain good performance at each extreme.

In miniature railway practice, a full size locomotive is usually selected to be modeled, so decisions regarding wheel arrangement, number of cylinders, etc. are already made.  Within the profile thus selected, the miniature locomotive designer must use his/her ingenuity to design the cylinders and boiler and prove the necessary water supply and fuel system.  The weight of the locomotive is always always well within the strength capabilities of the track.  However, wheel slippage is common with four coupled designs owing to the relatively small proportion of total weight carried by the driving wheels.  Because of this factor, drawbar pull is limited.

Six and eight coupled locomotives are less prone to slip than a four coupled configuration.  The boiler should be size so that it can generate an adequate supply of steam to the meet the high speed condition with a typical load.  A correlation between piston diameter and boiler pressure must exist, so that with modest regulator openings, the drivers will not slip.  This combination will give good slow speed control while maintaining adequate high speed performance.