TRACK DESIGN

1.       Carefully plan the radius of all curved track.

Follow the Free-mo standard in the design of all curved trackwork. While it’s obvious that major curves need to meet the standard, it’s easy to forget the minor ones. For example, be sure to plan the radius of the short curves that lead to turnouts at the ends of sidings and yard tracks. Examine all curves, no matter how short and inconsequential, to make certain that they conform to Free-mo standards.

 2.      Watch out for S-curves.

The following are some of the items that must be considered before construction begins.   Appendix 1 includes Gregg Fuhriman’s planning recommendations and also includes construction drawings he prepared for one of the Mojave Yard modules. Appendix 4 is a description of how I thought through the track design of Shandin Loop.

The Free-mo standard calls for 12” straight track between curves in opposing directions. Some track configurations, however, don’t permit conformance to the standard. The most common unaviodable non-conforming S-curves are located at turnouts leading to a siding that parallels the main track and at crossovers between two parallel tracks. Instances like these need careful attention since they create S-curves with very short straight track segments. If possible, alter your design so that non-conforming S-curves are avoided or use larger turnouts and the largest possible radius for the curves.

Keep in mind the actual radii of turnouts as you design your trackwork. The chart below shows selected data for turnouts conforming to the NMRA standards. Not all manufacturers conform to the standard!

In addition to what has been written above, it’s easy to forget that the diverging route of a turnout located at a module end may create an undesirable S-curve when mated to an adjoining module. Following the Free-mo standard, maintain the 6” straight track between the end of a module and the switch points of the first turnout on the module that is intended for use by mainline trains (e.g., a turnout leading to a mainline siding). NorCalF permits switch points that are closer than 6" from the end of a module if the turnout leads to a spur, yard lead or other track not intended for use by mainline trains, but other groups may not permit this.

Frog #	Radius of turnout	Length of straight crossover section
#6	43”	4-1/4”
#8	117”	5-11/16”
#10	117”	7-1/16”

 3.      Provide the necessary room for easements.

While easements make track and trains look graceful as they enter and leave curves and help to avoid coupler offset problems with long rolling stock, they do require extra space. A good rule of thumb is that the extra space required is one-half the length of the easement. While even short easements are helpful, plan easements that are as long as space on the module permits. Use one of the several good approaches to designing easements into your track plan (bent stick, templates, or plotting the actual spiral from formulas). If you want to get really serious about easements, the spiral easement calculator at The John Galt Line website is worth a look. The site provides an Excel tool that can be used to design and plot easements for any length easement and curve radius.

Whatever approach is used, I strongly recommend using the spiral easement calculator at The John Galt Line website to obtain the center point offsets for the constant radius curve. I recommend this procedure because, for example, the centerline offsets recommended by many bent stick articles are too large. Using this calculator also provides the spiral angle that marks the end of the easement and the start of the constant radius curve.

When using the bent stick approach, make sure that the stick used lies exactly along the tangent (straight) track to the point at which the easement begins at one end and exactly along the constant radius curve beyond the spiral angle that marks the point at which the easement ends. Doing so will insure that the easement is properly shaped.

 Dale Muir, in his in depth disucssion of the modeling of prototypical track offers this hint:

For multiple tracks, the inter-track spacing needs to increase on curves. I determine the length of spiral I want for the track with the bigger radius. Then work the formula backwards to determine the length of spiral needed on the inside track. The inside track spiral needs to be longer. This is actually a great way to disguise the fact that track centers increase on curves.

This approach to increasing the spacing of track on a double track curve was used with great success on Jere Ingram's Jastro module. Laying out the easements was greatly aided by track templates provided by Prawn Designs that included the tangent track, the easement, and the constant radius curve for each track.

 4.      Follow or exceed NMRA standards for track spacing on straight and curved track.

Doing this will provide clearance for long and overhanging locomotives and rolling stock.  Remember that prototype double track isn’t always spaced as closely as possible, especially for modern construction. Don’t forget to provide for prototypically articulated steam locomotives that overhang track centers by large amounts on curves. The NMRA standards that encompass typical Free-mo curve radii shown below should be considered absolute minimum track spacing. Err on the wide side whenever possible.

 

Inner track radius	Track center-line spacing
53”	2-1/4”
40”	2-3/8”
32”	2-1/2”

 Example: Track on a new, curved, double track module was spaced 2” apart. One setup participant had a prototypically articulated steam engine. The overhang of the engine was such that only one track could be used when the steam engine was on the inner curve. The owner of the module pulled the track up and re-laid it with increased track spacing to avoid this and similar problems.