Copyright 2000 T. Sheil & A. Sheil  All Rights Reserved

Standards for O27 and Semi-Scale

O27 and semi-scale - standards:

O27 has gotten the short end of the stick. Too many regard it as a "cheap train" rather than recognizing it as the first mass-produced scale train in America. Indeed, semi-scale was the concept that became O27. In subsequent years, the scale aspect has been diminished. Today, O27 means a smaller, cheaper O gauge train. For the sale of those who are "O27 purists," we got together and tried to resurrect the old standards. We doubt folks will give these standards absolute adherence. That is fine: this is a hobby where fun comes first. What we want to do is give O27 fans something on which to base their own appreciation of O27 trains, semi-scale and a better sense of toy train operations.

(In short, on Thursday night the editorial staff found itself with a lot of time and nothing else to do.)


Semi-scale is a 1/64 scale train operating on O trucks, wheels, made to run on O tracks. It may or may not be capable of handling a 27" diameter curve. We may occasionally describe it as O27 because it is semi-scale, even though it may be too long to handle the 27" radius curve.

True O27 is a 1/64 scale train operating on O trucks / wheels, made to run on O track, yet capable of handling a 27" diameter curve. True O27 fits within the definition of semi-scale.

Offscale O27 is a train, smaller than O, made to run on O track, yet capable of handling a 27" diameter curve. Offscale is not semi-scale.

Non-scale O27 is neither true nor offscale, but a train called by manufacturers or users "O27" merely because it can handle an O27 curve. Normally it is from an inexpensive line. Non-scale is not semi-scale. Normally it is the larger O gauge or 1/48 O scale.

Because of the wider track, a semi-scale car, locomotive or self-propelled unit may be wider than pure 1/64 scale. It may also ride higher, due to larger wheels and track.


Semi-scale trains are 1/64. However, it is common practice to incorporate 1/48 O scale scenery with 1/64 semi-scale O trains. We accept this as normal, and have therefore set the following general nomenclature for scenery:

Semi-scale scenery uses semi-scale trains with 1/64 scale buildings, accessories, etc. It is a 1/64 layout in every way, except for the gauge of the track. Our technical term is O27/64.

The use of 1/48 buildings, accessories and scenery with O27 trains is known as O27/48.

A blend of 1/64 and 1/48 elements is common. Therefore, the scale that predominates will determine if it is O27/64 or O27/48

Tin Scale is scenery that includes items notably larger or smaller than 1/64 and 1/48. Normally, these are "tinplate" items made to 1/32 scale or larger.


There is no true semi-scale track. However, there is O27 track, which blends better with semi-scale than O31 track. The standards for O27 track are as follows:

Curves: 27", 34" (Marx), 42", 54" and 72" The larger radii have the same height and width as the 27" rail, distinguishing them from O31

Standard straight is 8 3/4 to 8 7/8. We realize that some brands may be up to 1/4 inch off the original 13/16 inch standard.

Standard height for tubular rail is 3/8 to 1/2 inch, taking into account minor differences in manufacture.

The standard O27 track pin is narrower than its O gauge counterpart.

For sake of consistency, we can use the standards set by Lionel and Marx.

A note on scale differences:

Manufacturers have taken great liberties with their definitions of semi-scale, O and O27. In recent years, O27 as a term has been obscured by the growing number of offscale and non-scale items. In earlier years, some semi-scale items were marketed as O gauge. Some of these were too long to handle O27 curves easily. One humorous example is the Lionel 6-8-6 S2 Pennsylvania steam turbine locomotive. The #671 and #2020 are identical in size, but the #671 was sold as O gauge and the #2020 was sold as O27. A few minor differences in the motor placed the #2020 in a lower price range, hence its O27 status. Many steam locomotives sold as O gauge are in fact semi-scale, even if they cannot negotiate 27" curves.


The use of O27 track by Marx was a result of its use of a smaller train. These original trains were not semi-scale, but a smaller O gauge capable of handling smaller track. Lionel adopted O27 as its cheaper range of trains.

Semi-scale originated with American Flyer. The idea was that a long car or locomotive made to O scale (1/48) could have difficulty handling O31 track. Rather than have an O gauge that was offscale, American Flyer proposed a 1/64 scale train running on O track. This was the first true mass-produced scale train in America. Marx took the idea wholeheartedly, while Lionel used it partially. Semi-scale meant 1/64 scale.

When toy train production resumed after World War II, Lionel incorporated O27 with semi-scale to produce its cheaper range of trains. While many were true O27, others were a smaller offscale. Lionel also produced several long locomotives in semi-scale, but marketed them as O gauge. This is where the definition of semi-scale fluttered. Marx, on the other hand, produced semi-scale tinplate trains in its "scale" range, and then did the same in plastic. Marx retained the semi-scale standard for all but its 6-inch tinplate and 4-wheel plastic trains until closing. American Flyer produced S gauge after the war: a 1/64 scale train running on 1/64 trucks / wheels and 1/64 gauge track.

Since the 1970s, the definition of O27 and semi-scale have been obscured further. Many current O27 sets include scale cars and locomotives, the only link to the name being an ability to handle 27" curves. True semi-scale has suffered greatly. Semi-scale O27 should be a 1/64 scale train made to run on O track, and sold at a lower price than its O gauge counterparts. Indeed, O27 is the first scale train to use the O gauge of 1 1/4 inches.

Why set standards?

As O27 gets less and less attention, more adherents of semi-scale are seeking a way to bring O27 back into the spotlight. Standards remind us and others that O27 is not a cheap train, but a definite set of standards with precedents going back prior to 1938. Those who want more scale in toy train operations will find it within O27.

Copyright 2000 T. Sheil & A. Sheil  All Rights Reserved

Click here to return to the All Gauge Model Railroading Page, for the best free resources for model railroading and railfanning