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Introduction to S-MOD Railroading
The S-MOD System of Module-Railroading

(This introduction contains excerpts from the NASG S-MOD standards and from articles by Don Thompson and Don DeWitt, as published in the “S-Gauge Herald” and the NASG “Dispatch”. These standards and articles are copyrighted, and excerpts are used with permission from the authors and NASG.)

The basic idea of modular layout systems is that individual modelers can construct sections of layout which can be brought together and assembled into an operating layout. This is accomplished by adhering to physical standards (height and track alignment at the mating ends) and electrical standards (connectors, control scheme). In addition to being assembled into a large layout for local and national shows and conventions, such modules can be used at home as part of a larger layout, as a bookshelf“switching” layout, or as stand-alone units.

The S-MOD system has been designed to meet the above objectives and several others. 

-Design Flexibility - These standards do NOT include length, width, or corner standards in order to permit maximum design flexibility for the builder. There are however recommended practical size guidelines in the FAQ section of this site.Method of construction is also left to the builder, lightweight for easy transportation or heavy weight for sturdiness.

-Number of Tracks – Most SMOD modules have two mainlines, and many also have sidings.This is popular because it permits lots of action at train shows.The SMOD standards also allow for single track mainline, and there are many single track SMOD modules.Single track modules are gaining popularity due to the increased realism and design flexibility that they provide.The “Free-Mo” standards for single track modules have become quite popular, and “Free-Mo” webpages provide many good design ideas.The major difference between SMOD and FREE-MO single track standards is the floor-to-top-of-rail height.The advantage of following the SMOD standards for single track mainline modules is usage flexibility; the ability to connect your modules to other SMOD modules at train shows.

-Reversibility - The physical and electrical standards allow for reversibility of modules. The ability to reverse a module by turning it 180 degrees maximizes flexibility of use. 

-Flange size and rail height. - In “S”, as in other scales, there are a variety of wheel flange and rail heights in use. The standards do NOT specify rail size in order to permit flexibility for the builder. Trackwork built to NASG track standards S-3 will permit use of any rolling stock which has wheels and wheelsets that conform to NASG Wheel Standard S-4. The standards will also allow the use of American Flyer® and American Flyer® compatible (hi-rail) equipment provided that the rail is high enough (code 125 minimum). If you desire to run both AF and scale size wheelsets, closing frog switches must be used. 

-Electrical Simplicity - The basic wiring diagram shown at this site is simple, yet allows the module to be used in any layout, from the simplest oval to the complex layouts seen at national conventions.

Return to Main S-Mod Page

Revised September 17, 2002

Ted Larson

NASG Engineering Committee – SMOD 

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