TRACK DESIGN

 

The following are some of the items that must be considered before construction begins.  As an example, I've included in Appendix 4 a description of how I thought through the design of the 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 at the ends of side tracks.  Examine all curves, no matter how short and inconsequential, to make certain that they conform to Free-mo standards.

 

2.      Watch out for all 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 are at turnouts leading to a parallel side 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.  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 violates the Free-mo standard and may create an undesirable S-curve when mated to an adjoining module.  Keep in mind the radii of turnouts as you design your trackwork.  The chart below shows the following data for turnouts conforming the NMRA standards (NOTE:  not all manufacturers conform!):

 

·        The radius of the turnout closure rails.

 

·        If there is at least 1½” of straight track (for HO) before the switch point rails, the effective radius of the turnout.

 

·       The approximate length of the straight section of track for crossovers between tracks with 2”   centers.

 

                                Radius of             Effective       Length of Straight

     Turnout #          Closure Rails            Radius        Crossover Section

          #6                      43”                      56”                       5”

          #8                      67”                     110”                    6⅝”

         #10                    117”                      --                        --

 

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 rough rule of thumb is that the extra space required is one-half the length of the easement.  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, Dale Muir's AREA 10 Chord Spiral page is worth a look.  He provides a tool that can be used to design and plot actual spirals.  Also worth a look is a spiral easement calculator for use with the bent stick method.  There is a lot of detailed discussion of easements at this site as well.

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.  Also remember that prototype double track often isn’t spaced as closely as possible, especially for modern construction.  And don’t forget 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 when ever possible.

 

   Inner Track                 Track Center

       Radius                     Line Spacing

         53”                            2-1/4”

         40”                            2-3/8”

         32”                            2-1/2”

 

5.      Design turnout locations using the exact dimensions of the turnout to be used.  The track plan design MUST be prepared using EXACT "models" of the intended turnouts to be used since the various brands of turnouts are all different shapes and sizes.

 

Example:  The NorCalF Bay Shore yard ladder was planned using Walthers #6s, but it was built using Atlas #6s.  Unfortunately, since Atlas turnouts are slightly longer than the Walthers, the outermost caboose track does not have enough space to cross the endplate squarely and on the 2" spacing.

 

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.

 

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 provide for the Free-mo standard of flat and level scenery at the endplates.

 

 

4.      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.