ROADBED

 

I like to use cork for the roadbed.  After the track centerline is drawn on the sub-roadbed, it’s easy to carefully align one side of the cork with its center edge along the centerline of the track when gluing it down so long as adhesive is kept off the centerline.  Butting the other half of the cork against the first carries the centerline to the top of the cork roadbed.  At turnouts, each half to the cork roadbed can follow it’s respective track centerline, and the gap between filled with pieces of cork cut to fit.

 

1.      Use roadbed material that is resistant to expansion and contraction with humidity and temperature.  As with subroadbed, this is extremely important.  Failure to do so will introduce unpredictable results with trackwork itself and the scenery that surrounds it.  Ballasting and application of other scenery typically involves water-based glues which will cause problems with materials that expand when wet.  Sealed cork roadbed is a good solution.

 

2.      Glue rather than nailing cork roadbed in place.  Nails are likely to create dimples and dips in cork roadbed that may show when track is installed.  Use flat boards and weights to hold the roadbed down as the glue is drying.

 

3.      Roadbed must not introduce dips, rises and twists.  Glue must be evenly applied so that it doesn’t introduce rises and dips.  After the glue has dried, the installed roadbed can be sanded flat with a full piece of fine grain sandpaper wrapped the long way around a piece of 2” x 4” lumber.  Sand very carefully and only enough to insure that the cork is flat and level all the way across the module.  Be careful not to create any dips, rises or twists, especially at the end of a module where the sanding block isn’t fully supported by the roadbed being sanded.  Don’t forget to round off the rough edges of the cork so that the ballast slope is smooth, but be careful not to alter the flatness of the top of the cork roadbed when doing so.

 

Example:  One of the modules at a setup had a twist in the track that could be observed only when sighting along the track at track level.  One nicely detailed B-B diesel derailed regularly at that point.

 

4.      Changes of roadbed levels must be very gradual.  Transitions, for example, between main line track on HO roadbed and lower level side tracks or spur tracks on N or no roadbed, must be long and extremely gradual.  Very large radius vertical curves are required.  Keep in mind that 80-85 foot HO cars are about foot long.  Changes in level that are too abrupt will result in derailments, coupler mismatches or other problems.

 

Example:  A long, heavy passenger train derails as it exits a side track that is lower than the main track.  The derailment occurs as the rear coupler of an 85 foot car entering the slope leading to the main track would dips slightly.  The weight of the following cars prevents the coupler faces from sliding freely with the result that the truck of the leading car is lifted off the track and a derailment occurs.

 

5.      Superelevation on curves presents special problems.  If you plan to superelevate your curves, there are several things you’ll need to keep in mind:

 

·        It’s the outer rail that’s raised, not the inner rail that’s dropped.  The best way to accomplish this is to shim under the outer rail with strip wood when the track is installed.  Don’t try to build superelevation into the roadbed since it’s nearly impossible to do so successfully.  Remember that while prototypical superelevation may be as much as 5-6”, it is most often much less than that.  This amounts elevation of the outer rail over the inner rail of 1/16” at most.  Less than 1/16” is likely to be more prototypical, especially for more modern era modeling.

 

Example:  Out of curiosity, I tabulated the superelevation of all the curves in the ATSF Pasadena Subdivision in 1988 between MP 84 and MP 139.  The results are as follows:

 

                               Superelevation        Frequency        HO equivalent

                                      None                      5                 --

                                    ½” to 1”                   15                0.0115”

                                   1½” to 2”                  15                0.0230”

                                   2½” to 3”                  10                0.0344” (~1/32”)

                                   3½” to 4”                   6                 0.0459”

                                   4½” to 5”                   3                 0.0574”

                                   5½” to 6”                   0                 0.0689” (~1/16”)

 

·        The transition from level to superelevated track must very gradual.  Superelevation introduces a twist into trackwork that will result in derailments if the transition isn’t long and gradual.  If you superelevate, you’ll be asking the trucks of locomotives and cars to twist as they enter and leave superelevated track.  Prototypical transitions extend at least through the length of the easement (typically long) and into the straight track as required to maintain a gradual transition.  The elevated rail is held at a constant height throughout the constant radius curve itself.  One model trackwork expert recommends that the length of the transition equal twice the longest cars that will be operated.  In our case, that’s about two feet.

 

Example:  On a module incorporating superelevation, the length of the transition is short enough that some C-C diesels and some passenger cars frequently derail.

 

·        Unless you’re confident of your ability to do superelevation done correctly, skip it.  Broad, smooth curves and trackwork will often lead observers to think that track is superelevated when it isn’t.

 

6.      Drill holes for turnout actuators before laying track.  This takes careful planning but is impossible to do after track is laid.  An oversize hole makes certain that you have room for the switch machine actuating wire.  Gregg Fuhriman’s technique is best.  Drill a large hole in the subroadbed, lay the cork right over it, and then use an X-Acto knife or saw to make a narrow slot in the cork where the throw-rod of the turnout will be.  All this will work only if the exact location of the turnout throw-bar has been clearly marked at its exact eventual location on the subroadbed ahead of time during the centerline drawing phase.

 

7.      Pre-drill holes for track power feeder wires only if under-rail feeders are planned.  While this is the preferred method for many Free-mo modelers, I prefer to solder feeder wires to the outer side of rails after track is installed since planning hole locations for feeders before track is laid is hard for me to do successfully.  If you plan under-rail feeders, drill holes before laying track and make sure that they permit you to lay down the track exactly where you intend it to be.

 

8.      Seal the roadbed before laying any track.  This will extend the life of the cork or other material and will reduce if not eliminate shifting due to temperature or humidity.  Sealing with paint similar to the ballast color will also help disguise any areas where ballast is missed during application or damaged during handling.

 

9.  Prepare for IR sensor installation.  The optional Free-mo detection system for signaling includes IR detectors to provide detection of rolling stock that isn't detected by current detectors.  Installation of IR sensors is strongly recommended even if signaling isn't planned in the near future since it may become more interesting in the longer run.  Installation recommendations are included in Appendix 8.