Trackwork Handbook: Subroadbed
The subroadbed of Free-mo modules is typically the module top, whether extending across the full width of the module or some form of a “cookie cutter” approach. The module framework and subroadbed are the foundations upon which good trackwork depends. While invisible in the finished module, finished trackwork will inevitably expose each and every one of the framework’s and sub-roadbed’s faults. Whether made of foam, plywood, splines or other material, the framework and sub-roadbed must satisfy the following:
1. Use module frame materials that are strong, stable, warp free and resistant to expansion or contraction with humidity and temperature.
This is extremely important and will cause continual problems if ignored. Avoid is MDF! A material you can’t go wrong with is high ply-count Baltic or Finn birch plywood. It’s important to use material of sufficient thickness for the frame member involved.
2. The subroadbed must be flat, level and perpendicular to the end plates of the module.
This 3-dimensional requirement is absolutely fundamental to successful module creation.
1st dimension: End plates must be exactly perpendicular to the subroadbed. It’s essential to read and follow Gregg Fuhriman’s advice in Appendix 2.
2nd dimension: The subroadbed (usually the module top) must be level from side-to-side with the endplate. At a setup either the track or the end plate will be used to determine of a module is level. If the subroadbed isn’t exactly level with respect to the top of the end plate, the track won’t be level with the end plate at the end of the module either. Out-of-level subroadbed will force a twist in the track at the bridge rails or make it impossible to match up the endplates nicely with adjoining modules.
3rd dimension: The subroadbed must not slope down or rise up at the module end. The most common module design has the top of the module extending to, but not over, the top of the endplate. Any mismatch of the module top and the top of the endplate will likely result in one or the other of these problems.
If the top edge of the endplate is high relative to the module top, the only solution is to reduce the top edge of the endplate. Failing to do so will result in a rise in the track at the end of the module.
If the top edge of the endplate is low relative to the module top, there simply isn’t a good solution. Sanding the module top to match the endplate will result in a dip at the module end that will be reflected in the track.
Example: The distance from the pilot and the front truck kingpin on an Alco PA-1 and some other engines is long enough that the pilot moves up and down quite a lot as the locomotive travels through dips and rises in trackwork. At a “V” shaped dip at a module joint, metal pilots or plows contact the track and cause a short.
Example: The use of prototypical size couplers on long cars can result in uncoupling as the couplers rise and fall relative to each other on track that isn’t level through the bridge rails.
3. The subroadbed at each end of the module must be level with the roadbed at the other end.
Any twist of the subroadbed from one end of a module to the other will result in twist in the track at the bridge rails at one end of the module or the other or will force twisting of the whole module at the setup.
Example: At a recent setup, a twist at the bridge rails resulted from a difference in side-to-side leveling of the subroadbed even though the endplates were level with each other. The result was that some C-C locomotives would derail since the twist across the bridge rails was so short and so pronounced.
4. Unless there is a grade included in the module, the sub-roadbed should be flat and level across the length of the module.
Again, this means truly flat and truly level. Use a straight edge and sight along the module to make sure that the sub-roadbed is truly flat. Support the subroadbed well and securely along its length to make sure that it will stay flat and level over time. Many modules have lasted 10 years or more and have been subjected to a lot of handling and transportation. Some have held up well; others haven’t done so well.
5. If there is a transition to a grade included in the module, employ vertical curves that are extremely gradual (very large radius) between the level and the inclined track.
Too abrupt a transition will result in derailments, mismatched couplers, and will look un-prototypical. The steeper the grade, the longer the vertical curve needs to be. If in doubt at all, make vertical curves of extremely large radius. The Northern California Free-mo recommendation for vertical curves is described in Appendix 5.
Remember that transitions in track level (e.g., from HO roadbed to N roadbed or to the subroadbed) are grades and must be long and gradual. See item 4 in the following section on roadbed.