Computer Modelling

The deviation from the minimal size, which is a circle, is that two constant radius modules have been replaced by two transition and two easement modules; this allows for later simple insertion of straight modules to yield an oval.

The image shows eighteen module baseboard outlines, the centreline of the primary track (this is a two-track railway), and a bounding box with tick marks at one foot intervals.

Railway module and track dimensions are not specified in terms of the track radius. The primary reason is to minimise material waste - which may be surprising to read.

The explanation is that track and rail normally are sold in fixed lengths (6 feet in the USA, which will be used in this example). Thus, the computer code is set with an arc length for the primary track centreline for a module to correspond to this fixed length. In the target initial railway shown this arc length is 71.0 inches. The outer primary gauge 1 track rail length is 71.35. Thus about 5/8 inch is available for trimming the ends of the rails on installation. It should be added for clarity that an arc length is module-specific: it is not required that all modules have the same arc length - although, in this example, that happens to be the case. The reason for the flexibility is that the computer code that handles railway design is based on arbitrary two-dimensional vectors.

Another parameter that it is useful to control during railway design is the angular change made by a curved module, named the turn in the computer code. Module curvature is specified in the computer code by the number of modules that form a complete circle; and this then specifies what is called the module turn. Once the arc and the turn are specified, the radius is an arithmetic result.

The target initial railway in the image comprises 14 modules with constant curvature; these are 16mpc (16 modules per circle) with arc length 71.0 inches, plus 2 transitions, and 2 easements from the constant curvature to zero curvature (straight). To see how this works out so nicely see Application. The modules are two track with the primary track on the outside; this results in the (non-existent) inner secondary track arc length being smaller than the primary track arc, which is desirable from a manufacturing point of view because of the 6 feet track supplied length. Since the base modules are 16mpc, the turn of one module is 22.5 degrees, and the centreline radius of curvature of the primary track is 180.8 inches, or 15.07 feet.

The computer code is not unit-specific except for a conversion value of 12 to enable interchange between feet and inches. Other than this, easily changed, value, there is no reason why numbers cannot be interpreted in, for example, the metric system.