Coupler Size: Prototype and Model, by Bruce A. Metcalf

Introduction


Figure 1: AAR Type E	Figure 2: NMRA S-6/RP-21.2

One problem often faced by modelers, particularly those in new or minority scales, is the selection of appropriate couplers. There is presently no NMRA Standard for couplers in any scale. The X2f or "horn-hook" design was never accepted by the HO membership, and the former Standard for solid (dummy) couplers was reclassified as a Recommended Practice in 1957.

However, because of the urgent practical need for a standard coupler, the product lines now manufactured by Kadee and Micro-Trains became de facto standards in most modeling scales. The recent rush of competitors in HO scale have thus restricted themselves to copying the Kadee profile for reasons of compatibility.

That said, the large range of products now available makes selection difficult for those who want to balance accurate scale size against inter-operability with their fellow modelers, especially in the "large" scales running on 45-mm gauge track. (Proper names for the various "large" scales and gauges were discussed in my article "Scale and Gauge: A Response" in NMRA Bulletin, October 1991, p. 40.)

Perhaps the best way to begin the decision process is to review the sizes of prototype couplers. While modern knuckle couplers have inherited more than a century of compatible development, not all prototype couplers are the same size. As we have to know the size of the prototype before we can intelligently select a size for our model, prototype size warrants some study.

The first problem is to define what we mean by "size." Prototype (and model) coupler bodies vary significantly from one manufacturer to another, and from one manufacturer's product over the past century of production. Furthermore, model couplers aren't always able to have the same proportions as prototype couplers for practical engineering reasons.

Having to select a single dimension for comparison, I elected to choose one that gives the impression of correct size when viewed on the layout. Given the widespread use of high benchwork by careful modelers, (see my remarks in "Model Railroad Benchwork," originally published in The Columbus Clinics, 1992, especially the sidebar on p. 36.), I concluded that couplers are most commonly viewed from the side, as shown in Figure 1 and Figure 2.

Knuckle height is one of the few prototype dimensions visible from the side that can also be easily measured in the model. Thus this study makes the necessary assumption that knuckle height is the appropriate definition of coupler size, though I concede that this point may be worthy of debate.