Trackwork Handbook: Roadbed
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.
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. I have been
happy with the appropriate Liquid Nails adhesive.
Uneven application of adhesive can result in bumps in the roadbed. Gregg Fuhriman recommends using an adhesive spreader to assure that the adhesive is applied evenly. Following his advice, I use an inexpensive spreader with 1/16” x 1/16” teeth. Keeping the teeth clean is important so that the layer of adhesive is adequate. Use this spreader when spreading adhesive for track as well.
3.
Roadbed must not introduce dips, rises and twists.
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 12” long flat piece of 2” x 3” or 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 large setup had a twist and angular rise in the
track partway through the module 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:
The
first car of 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 the car would
naturally dip as the car enters the track leading up to the main track.
Because the following train is heavy, the rear coupler resists
sliding against the coupler of the following car, the rear truck of the
first car is lifted off the track and a derailment occurs.
If the couplers were to slide freely, an uncoupling is possible.
5.
Superelevation on curves presents special problems.
If you plan to superelevate your curves,
there are a few things 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.
Example:
Out of curiosity, I tabulated the superelevation of all the
curves in the ATSF
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 track, 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.
For HO, 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 incorporate superelevation
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 actuating wire 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 I don’t recommend it, this is the
preferred method for some 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 difficult 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.
I prefer to solder feeders to the outside of rails. There’s good advice on track feeders (and other subjects as well) in Easy Model Railroad Wiring by Andy Sperandeo.
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 with
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 occupancy detection
system for signaling includes IR detectors at a block boundaries to
provide detection of rolling stock that won’t be detected by current
detectors. Installation of
IR sensors is strongly recommended even if signals aren’t installed
initially. Installation
recommendations are included in
Appendix 8 along with a
more complete description of the occupancy and detection system.