People who do not collect or operate toy trains and model railroads often do not understand the different sizes and scales that are available.  This guide can help you to determine what you have so that you can list it in the correct category.

[NOTE: This was originally written to display with a fixed width font.  It has been "adjusted", but the table and diagrams may not "line up" perfectly on your display.]

                COMMON MODEL RAILROAD GAUGES

    SCALE or    SCALE   SCALE DIMENSIONS     TRACK GAUGE
     GAUGE      RATIO      (per one foot)             DECIMAL  METRIC
     NAME                     DECIMAL   METRIC

       Z             1:220       .054"         1.37mm         .256"      6.5mm
       N             1:160       .075          1.9               .354        9.0
       TT            1:120       .100          2.54             .472      12.0
       HO           1:87.1      .138          3.5               .650      16.5
       S             1:64         .1875        4.76              .875      22.2
       O             1:48         .25           6.35            1.25        31.75
       #1           1:32         .375          9.5              1.75        44.45
       G             1:24         .500         12.7            1.77        45.0
       STD           --            --             --                2.125      54.0

Measure the gauge between the OUTSIDE of the flange on wheels (1),or between the INSIDE of the outer rails (2). (In the case of 3-rail track, measure between the "running rails" -- the rails on which the wheels run.)  NOTE:  The measurement on the wheels will be slightly less than the "TRACK GAUGE" shown above.  This is to allow for necessary clearance.

                    ._/\                        /\_.
                    |   |                        |   |
                    |   |                        |   |
                    |   |------------------------|   |
                    |   |============|   |
                    |   |------------------------|   |
                    |   |                        |   |
                    |_ |                        |_ |
             (1)--->\/                         \/<----(1)

                     O<----(2)  O   (2)---->O
                     ||             ||              ||
                   ./-\------------/-\------------/-\.
                !====================!

Prototype "standard gauge" is 4' 8-1/2".  Dividing this by the model gauges and comparing the results to the "ratios" above make it clear that model gauges are not exactly to scale.

Z, N, TT and HO trains are often reasonably accurate scale models.

Actual scale used for S, O, #1 and G gauge trains varies greatly, often depending on whether a true scale model or a toy train is intended.

A scale ratio for model "standard gauge" has never been established. It might be in the area of 1:28.  Most "standard gauge" trains are regarded as toys and are not made to a realistic scale size.

In prototype (full size) trains, "standard gauge" refers to trains that run on track with 4' 8-1/2" between the rails.

In model/toy trains, a train that runs on track which has a scale size of (approximately) 4' 8-1/2" between the rails are called "standard gauge".  This use of "standard gauge" differentiates between these trains and those that run on narrow gauge tracks or on wide gauge tracks.  (2-1/2 or 3 feet, and 1 meter are typical "narrow gauges"; 5 feet is a typical "wide gauge".  Wide gauge is rare, while narrow gauges are more common.  Narrow gauges provide a way to build a right-of-way that is considerably less costly for light duty use.)

Confusing the issue is the "standard gauge" toy train size. This uses track that is 2-1/8" between the outside rails. As stated above, there is no established "standard scale".