Boulder Model Railroad Club HO Module Specifications

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Last Modified On: Monday, 12-Apr-1999 20:55:44 MDT


These specifications and plans describe the standards used in constructing HO modules by the Boulder Model Railroad Club (BMRC). They are designed to provide an introduction to modular railroading for the beginner, as well as provide the standards which are used by the BMRC.

These standards are used to ensure compatibility between different members' modules and trouble free operation of the assembled layout as a whole. However, since one of the advantages of modular railroading is to allow each member to 'personalize' their own module(s), some variations can be allowed, as long as they are approved in advance by the module coordinator!

The setting for your module is entirely up to you. It can be modern or historic, urban or rural, mountains or desert, etc. There are a few standards regarding backdrop color, etc., which are listed on the next page, but otherwise almost anything goes. In particular, terrain which goes below the track level to allow bridges, trestles, etc. is permitted (even encouraged), but any cutting or modification of the module framework must be approved by the module coordinator. Tunnels on the mainline are discouraged - remember that many of our members run modern equipment with 3 level auto-racks and double stack containers, so any tunnels MUST have adequate clearance.

A couple of things to think about in designing the scene for your module: First of all, since the primary purpose of these modules is to be on public display, any eye-catching or humorous elements in the scene on your module are a plus, particularly things which imply activity other than the trains moving through the scene.

Another thing to consider is that your module is going to be subjected to rougher treatment than your layout at home, no matter how tenderly and carefully you treat it. This means that your scenery must be even more solid in its construction. Buildings, and especially figures, must be anchored down much more solidly than you would on your home layout (some people even make the buildings on their layouts removable, and carry them separately when transporting their modules). Any kind of mechanical animation on a module is a real crowd pleaser, but transporting it can throw it out of whack before you know it. If you want to have something like that, consider making it removable so that you can transport it separately (not to mention being able to perform maintenance on it if something does go wrong).


Please remember that while individual creativity and craftsmanship are encouraged in building your module, when these modules are out on display for the public, their purpose is to provide an entertaining display with continuous operating trains. If there is some element of a module which interferes with the operation of the trains, especially if it is something that is a departure from these standards, then any necessary corrective action to get the show 'back on track' will be taken, Such action may include (at the module coordinator's discretion): repair of the problem (if it can be done quickly), removal of whatever is causing the problem from the module (for example, removing a bad turnout and replacing it with a length of flex-track), or removing the entire module from the layout. Please keep this in mind when designing and building your module(s).

These specifications are for the 'standard' 4 foot straight module. For information on other types of modules (such as corners), or on options such as narrow gauge track, contact the module coordinator.


Those standards complying with NMRA S10 for HO scale are marked with an *.

Module Dimensions

Abbr. Name Value Meaning
ML Module Length* 48" The length of the module from one end (interface) to the other
MW Module Width1 32" The width (depth) of the module from front to back
ME Module Elevation 44" The distance from the floor to the top of the mainline rails

Skyboard (Backdrop)

Abbr. Name Value Meaning
SH Skyboard Height*

4" The distance from the top of the interface (from the end board) to the top of the skyboard
SL Skyboard Length 47-7/8" The length of the skyboard (1/16" less than the Module Length at each end)
  Skyboard Color Komak P18-16 The color to paint the sky on the skyboard (other features such as clouds or distant mountains are up to you)


Abbr. Name Value Meaning
SB Track Setbacks 1 6", 8" The distance from the front of the module to the center line of the two mainline tracks
TC Track Centers* 2" The distance between track center lines on tangent (straight) tracks
2-3/8" The distance between track center lines on curves
MR Minimum Radius* 40" The minimum radius of curves on the mainline tracks. Provide 1/4" offset easement into curves (see diagram)
30" The minimum radius on non-mainline curves
BA Bridge Track Allowance 4-1/2" The length of track left 'clear' at the end of each mainline to allow installation of the 9" 'bridge track' between modules (see diagram)
IL Interface Tangent Length 5-1/8" The distance in from the interface (end of the module) that the mainlines must remain straight (including the Bridge Track Allowance) before any curves begin (see diagram)
  • All applicable NMRA standards shall govern trackwork
  • Track may be either prefabricated (e.g. Flex-Track), or hand laid. In either case, rails must be nickel-silver (NO brass!)
  • Rail must be code 100 on the front main, and on all bridge and IL tracks. The rear main (between the IL tracks) and all other trackage may be code 70, 83, or 100
  • No grades allowed on either mainline
  • Minimum mainline turnout: no. 6.
    Minimum non-mainline turnout: no. 4
    No plastic frogs allowed
  • Mainline ballast color: medium gray blend
    Non-mainline ballast: Up to you.

1 taken together, MW and SB do comply with NMRA S10.


Name Requirement
Wiring Harness The primary wiring harness for the mainlines must be 16 ga. wire (minimum), with Molex plug connectors (male at the east (left) end and female at the west (right) end).
The connections to the rail are as follows:
Pin #1 - Rear rail of rear main (the one farthest from the viewing side)
Pin #2 - Front rail of rear main
Pin #3 - Rear rail of front main
Pin #4 - Front rail of front main (the one closest to the viewing side)
Connections All connections must be soldered or use screw type terminal strips
No common rail wiring
Each rail must have its own feeder (18 ga. minimum). For even greater reliability, some people supply each rail with two feeders; one at each end of the module.
Other Electrical Elements Additional electrical elements, such as trackage capable of operation independent of the mainline, lighting, or animated displays are great eye-catching crowd pleasers, and are therefore encouraged. However, it is the responsibility of the individual module owner to provide any necessary power supplies, extension cords, etc. While every effort will be made to accommodate the needs of such modules (especially if the module coordinator is given advance notice), there is no guarantee that the necessary hook-ups will be available at every show. Therefore, it is important that the main lines be able to operate even if the connections for the 'extras' are not available.


Name Requirement
General Construction All joints on the module framework MUST be fastened together with glue (white, carpenter's, etc.) and either screws (preferred) or nails. All corners should be checked with a square and all surfaces of the assembled module (except Masonite) should be sanded to improve appearance and avoid slivers.
Good woodworking practices should be used throughout in the construction of the module. Remember that it is going to get a lot of use and travel many miles, so it must be solid to begin with. If you have any questions regarding construction, or need assistance, please contact the module coordinator.
Masonite Front
Fascia & Skirting
The Masonite front fascia and skirting are to remain the natural Masonite color. You may put on a clear protective coating, but do not paint. Labels, heralds, and other identification are OK, and might even be good to have if they enhance your module, but do it neatly. Remember that these are for public display.
NOTE: Use only Tempered Masonite, which has much greater strength.
Testing It is much easier to find and correct any problems with your module as you build it, and not when you get it all put together and on display at your first mall show! Test everything, especially track and electrical work, as you go.


Appendix A: BMRC HO FoamRail

This appendix to the original specification describes a new set of standards which can be used to construct an extremely lightweight and portable module that is compatible with existing BMRC HO modules described in the previous HO Modular Layout Specifications. This new standard has a dramatically simplified construction and will be transportable in a typical compact car. To also reduce the effort and time required to setup the modules, legs have been eliminated and the HO FoamRail modules will require support from existing BMRC modules or be placed on top of a table. In Appendix B, there are diagrams for support stands which allow FoamRail modules to connect to each other and previous modules.

Note that these foam module specifications are a super-set of the existing BMRC HO Module Specifications. Please refer to those specifications and then note additions or differences in the HO FoamRail specifications below.

This specification only describes the construction of a "straight" module. Corners have not yet been specified, but usage of existing corners or the BMRC return-loop modules will be required to construct a continuous running layout. This specification is an extension to the previous HO Modular Layout Specifications, deviations are noted below.

Foam Module Dimensions

Abbr. Name Value Meaning
ML Module Length 45" Length shortened to ease construction. BA+(36" flextrack)+BA
MW Module Width 24" Width reduced to ease construction and reduce waste of expensive foam. Typical sheets of 2" thick foam are 2'x8' or 4'x8'.
ME Module Elevation NA Legs are not defined for this module specification; however, see next appendix.

Skyboard (1/4" foamcore backdrop)

Abbr. Name Value Meaning
SH Skyboard Height 14" Distance from top of interface to the top of the skyboard
SL Skyboard Length 44-7/8" The skyboard is centered length-wise on the module so that there is a 1/16" gap at each end to allow space variations between connected modules.
  Skyboard Color Komak P18-16 Backdrop scenery painting is allowed, but be careful not to detract attention from module scenery to the trains!


Name Meaning
All specifications Remains unchanged from previous HO module specification. BUT use of un-cut 36" code-100 Atlas flextrack is highly recommended for both tracks.


Name Requirements
All specifications Remain unchanged from previous HO module specification.
NOTE Wiring harness should extend at least 12" past each end of the module to allow connection to HO modules built according to the previous specifications. The Molex connectors can also be found by purchasing a standard Personal Computer disk drive power Y-cable. It is strongly suggested that one make the connector more sturdy by cementing the wire exiting the connector with epoxy. Be careful not to allow epoxy to foul the connector mating points.


Name Requirement
General Construction The foam within a module may be fastened together with latex contact cement, white/yellow glue or some other latex based adhesive. These adhesives may also be used to fasten the Masonite edges and clamping pads underneath the module and the cork roadbed.

It is strongly suggested that white glue (not contact cement!) be used to fasten the trackwork to the cork roadbed and short track nails can be used to temporarily hold the track in position. Be careful to place the track according to the specifications.

If the module is connected to other foam modules, long woodworking clamps (provided by the module owner) must be used to fasten the modules together.

The 1/8" Masonite edging is glued to the module with 1/2" extending below the bottom of the module. This will protect the edge of the module from damage and will allow easy setup on top of tables when the modules are connected at a show. At the interface between two modules, two L-shaped wooden blocks (provided by the module coordinator) will be placed under the front and back edging to support the modules slightly above the top of the table. This will allow more space to clamp modules together and will make the modules less susceptible to rocking on uneven table tops.

The skyboard may be permanently attached to the module or may be temporarily attached using Velcro strips. Small L-shaped pieces of skyboard will be provided by the module coordinator to fill the 8" gap between non-foam and foam modules.

Under Investigation
  • Skirting to hide the hole underneath foam modules when used with earlier HO modules.
  • Inside corner module specification to allow L-shaped module layouts
  • Outside corner module specifications to replace existing corner modules.

Bill of Materials

Qty. Description Usage
1 2'x8' high-density extruded foam 2" thick (NOT beaded foam board!) Piece 45" by 24" is used as top portion of the module. 4" wide pieces are cut from remainder to create stiffing structure (see BOTTOM view) attached below top piece. Remaining foam may be used as scenery material.
1 14"x44-7/8" skyboard from 2'x4' sheet of 1/4" foamcore Paint both sides with specified color to prevent warping.
2 Atlas code 100 flextrack (36" length) No cutting required! Use of code 83 track is allowed on inside track. Refer to module specifications for details.
3 Midwest cork roadbed Minimum required for two straight tracks. Switches and extra track will require more roadbed.
2 4-1/2"x45" of 1/8" Masonite with one tempered side Outside edging.
2 2"x16" of 1/8" Masonite with one tempered side Clamping pads used to protect foam when clamped to other modules.
2 4 lug terminal strips (Radio Shack) Attach one to each end (east and west). This is used to provide an easy place to solder wires and connectors together,
2 4 pin Molex connectors (1 male, 1 female) Be sure to leave 12" of wire between end of connector and the edge of the module.
misc 25' of 16/18 gauge stranded wire,
Adhesive for foam, cork roadbed,
Refer to specifications for details
2 2"x2" strips of Velcro hook Used to connect foam modules together
2 Atlas 9 " Snap track (part no. AT-150) Bridge tracks used to connect modules together
2 9"-12" woodworking clamps Used to connect foam module to existing wood modules and to connect foam modules (on table top) together.

Figures for Appendix A:

Appendix B: BMRC HO FoamRail Support Stands

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