Trackwork Handbook:  Track Design

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

 

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.

While the Free-mo standard calls for 12” straight track between curves in opposing directions, some track configurations don’t permit conformance to the standard.  The most common S-curves are 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 very tight S-curves with very short straight track segments in between the curves.  If possible, alter your design so that S-curves are avoided or use the longest turnouts and the largest radii possible for adjoining curves.

 

Also, 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.  If possible, maintain the 6” straight between the end of a module and the switch points of the first turnout on the module.

 

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. (NOTE:  not all manufacturers conform to the standard!):

 

                Frog                    Radius of           Length of Straight

              Number                  Turnout            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 also 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 (registration required), or plotting the actual spiral from formulas).  If you want to get really serious about easements, Dale Muir's AREA 10 Chord Spiral page or the spiral easement calculator at The John Galt Line website is worth a look.  Each of these sites provides a tool or approach that can be used to design and plot easements.

 

My personal recommendation is to use the spiral easement calculator at The John Galt Line website to obtain the center point offset for the constant radius curve and then use the bent stick method to plot the easement.  I recommend this procedure because the centerline offsets of many bent stick articles (including the one referred to above) are excessive.

 

When using the bent stick approach, make sure that the stick used lies exactly over both the tangent (straight) track and the constant radius curve at the ends of the easement so that the easement is properly shaped.

 

Dale Muir, author of the AREA 10 Chord Spiral page, offered 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.

 

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.  And 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.

 

                                                              Class 1a

                    Inner Track                       Track Center

                       Radius                           Line Spacing

                         53”                                 2-1/4”

                         40”                                 2-3/8”

                         32”                                 2-1/2”

 

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

 

5.    Design turnout locations using the exact dimensions of the turnout to be used. 

The track plan design must be prepared using exact drawings of the intended turnouts to be used since the various brands of turnouts are all different shapes and sizes.  For the Mojave Yard, we used schematics that included the location of the ends of the switch rails, the location of the throw bar and the centerline of the through and diverging track.

 

 

6.    Make sure module structural elements aren’t below switch machine locations, track power feeders, etc.

This is easy to do when planning and extremely difficult to fix later on.

 

Example:  A module was initially built with a hand operated switch stand on the top of the module.  Later installation of a switch machine under the module was impossible because a structural support for the roadbed was directly under the throw bar of the turnout.

 

7.   Decide on roadbed and track profile ahead of time.

Decide what profile your track will have as it traverses the module.  Is it flat with the ground surface as in yards?  Is it ballasted?  Do the ties and ballast rest on a subroadbed?  Are there drainage ditches on each side of the track?  Are the scenery slopes on either side of the track consistent with the nature of the geology being depicted?  Does it all look as though it was created by a prototype railroad engineer?  Avoid profiles and adjoining scenery that looks like it was designed by a modeler trying to force the most impressive scenery into the limited available space.  The sketch below is based on 1939 A.R.E.A specifications and might help get you thinking[1].  A sample of actual roadbed, ballast and track profile drawings can be found in Appendix 6.  Remember to conform to the Free-mo standard that requires a flat and level scenery profile at module endplates that is 3/8” below the rail tops.

 

 

8.    Work through a complete design checklist before purchasing materials and beginning construction.

Once you’ve purchased materials and begun module construction, it’s very hard to back up and correct any omissions in planning.  Study the great checklist prepared by Gregg Fuhriman in Appendix 1.

 

Gary M. Green, Aptos, CA 95003
E-mail:  ggreen@inreach.com