Modules: S-MOD

The NASG S-MOD Standards were developed and approved in July 1987. This standard allows S-scale modelers to gather from anywhere and easily join their modules into functioning, hassle-free layouts. These standards have been accepted by the NMRA Engineering Committee as the basis for the NMRA's S-scale module standards. They accommodate standard-gauge, narrow-gauge, and/or hi-rail. Track and wheel dimensional standards match those described on our dedicated Standards page.

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Official Documentation


The first document describes the physical construction requirements for a module to meet the NASG's S-MOD standard. Note that this standard's requirements are kept to an absolute minimum, so as to allow for maximum flexibility and freedom of implementation, yet still allow for random modules' integration into functional layouts. This standard allows for closed-loop as well as point-to-point layouts, or a combination of both.

NASG S-MOD Physical Module Standards (PDF).


After the physical construction, the next most important aspect of getting modules to work together is the electrical sub-system. The document below describes the accepted recommended practice for creating the electrical sub-system for S-MOD modules.

Important: the electrical design was created in the late 1980s. At that time Digital Command Control (DCC) wasn't as common as it is now. The NASG BOT is currently (2020) working on a revised design that will allow for DCC. Also note that the interconnecting plugs referenced in the document below, the famous Cinch plugs, are no longer available, and neither is the company through which everyone bought these plugs, Radio Shack.

NASG S-MOD Layout Electrical System (PDF).

Below is a follow-up article written by Don DeWitt describing how to check and verify the wiring of your module.

NASG S-MOD Layout Electrical System (PDF).


As a source reference, the following NMRA documents cover the standards for modules in all scales, including S.

Supplemental Articles

The key drivers behind the S-MOD standard, Don Thompson and Don DeWitt, wrote a four-part series of articles for the S Gauge Herald magazine on how to construct S-MOD modules following the above-referenced standard. These have been made separately available below. The first article covers various designs that are possible with the S-MOD system.

  1. Planning the Module (PDF)
  2. Building the Benchwork (PDF)
  3. Laying the Track (PDF)
  4. Installing the Electrical System (PDF)


1990 NASG Convention

Probably the largest gathering of S-MOD modules ever was at the 1990 NASG Convention in Pittsburgh, PA. The final track plan of this layout is shown below. For more details about this, see the August 1990 NASG Dispatch issue. Click the image below for a larger version.

Houston S Gaugers

Probably the largest set-up the Houston S Gaugers were able to accomplish was at the 2014 World's Greatest Hobby show in Houston, Texas in January of 2014. Over thirty modules, all built to the S-MOD standard were convened together at this show. The George R. Brown convention center in Houston has an upstairs area with large portal windows that allow viewing of the show floor below. It was from this vantage point that this photo was taken. Unfortunately, it still wasn't high up enough to capture the entire layout (the right-hand edge of the layout could not be captured). The photo was taken late in the day when the crowds had died down a bit, allowing for time to visit the various vendors and for taking of this photograph. Club members Ria (left) and Bob Werre are seated in the center of the layout, while trains run on the layout. There were as many as 6 full trains running on the layout at one time. The club was allocated a 26' x 70' space. Set up took many hours on the Friday before the two-day show. Club member Peter's train is parked in a siding in the center-left of the photo (headlights on). Club member Steve (blue shirt, center-top) is running a steam train (visible in the top-right of the photo).

copyright © Peter Vanvliet; used by permission

Double-track wye modules, built by Jack Troxell and completed by Peter Vanvliet; copyright © Peter Vanvliet; used by permission)

North Penn S Gaugers

This is the scene of a module on the North Penn S Gaugers club layout.

copyright © Hugh Sinn; used by permission


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