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.
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.
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 non-conforming 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 S-curves with very short straight track segments between the curves of the turnouts. If possible, alter your design so that S-curves are avoided or use longer turnouts and the largest radii possible for the 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. 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
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.
Number Turnout Crossover Section
#6 43” 4-1/4”
#8 117” 5-11/16”
#10 117” 7-1/16”
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
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 and the
spiral easement calculator at The John Galt Line website
are worth a
look. Each of these sites
provides a tool that can be used to design and plot
easements. (Java security settings may need adjustment to run the
AREA 10 Chord Spiral tool.)
(Java security settings may need adjustment to run the AREA 10 Chord Spiral tool.)
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
recommended by many
bent stick articles are not
Using this calculator also provides the spiral angle that marks the end
of the easement and the start of the constant radius curve.
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.
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 that inlcuded the tangent track, the easement, and the constant radius curve for each track that was provided by Prawn Designs.
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.
Radius Line Spacing