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

Prototype Couplers

In 1887 the Master Car Builders Association (MCB) adopted a standard design for a Janney-style knuckle coupler very much like the ones in service today. In fact, if one were to be installed on a car it would couple with the newest designs. Two years later, the height of the knuckle was fixed at 9 inches [about 23-cm].

In 1904 this design was modified slightly and was designated the MCB5, the "5" reflecting the use of their #5 contour (Figure 3).

By 1916, enough road testing had been done that the MCB adopted their Standard Type D coupler in that year, although the #5 contour of the 1904 MCB5 design was, at first, retained.

1918 saw the adoption of the new, #10 contour. It had proven itself compatible with the #5 contour and the Type D, also designated MCB10, dates from that decision (Figure 4).

Figure 3: AAR Type E	Figure 4: NMRA S-6/RP-21.2	Figure 5: Type E Coupler

The MCB became the American Railroad Association (ARA) in 1918, and later yet changed their name to the Association of American Railroads, by which name they are known today.

In 1930, the ARE replaced the Type D coupler with the Type E. The Type D was not to be manufactured after 1934. The contour of the new coupler was designated #10A, and differed but slightly from the #10. Again, full compatibility was maintained (Figure 5).

Many narrow gauge and industrial lines, especially those not subject to regulation, preferred smaller couplers more appropriate to the lighter loads (and budgets) of those lines. Couplers were produced that were ¾ and ½ the size of the MCB5. Their knuckles' heights were 6¾" and 4½" [17-cm and 11-cm], respectively.

The ¾MCB couplers were used on the WW&F, SP, and CN narrow gauge lines, and they remain in daily use on the Disney railroads. Heavy narrow gauge lines like the D&RGW and C&S used standard size couplers on their narrow gauge equipment for compatibility with standard gauge equipment with which they sometimes inter-operated (through truck swapping or other such expedients).

The ½MCB couplers usually found service on captive industrial lines or 2-foot narrow gauge operations. The differences between the MCB5 and the MCB10 apparently did not warrant retooling, and by the time of the arrival of the Type E such shortline operations as survived had largely begun the shift to full-size couplers. Thus no "¾-Type D" or "¾-Type E" couplers appear to have been produced.

The prototype engineers who worked on coupler design were primarily concerned with coupler contour, as this is what determined if two dissimilar couplers could reliably mate with one another. The height of the knuckle, and that of the coupler body, were of lesser concern and thus were often omitted from discussions of coupler dimensions.

While there is therefore some ambiguity about the size of the 1889 design, the MCB5 and Type D couplers used knuckles that were 9 inches [23-cm] in height. The Type E introduced in 1932 used a knuckle 11 inches [28-cm] tall.

Two other coupler types are worthy of mention, though I am not aware of anyone modeling them in any scale (except for a few museum models): The Type H Tightlock coupler was adopted as an AAR Standard in 1947 (Figure 6). It was meant primarily for use on lightweight passenger cars where slack caused an uncomfortable ride and excess noise, but it also helps to prevent overturning in case of derailments and is now required on nearly all passenger service equipment.

The Type F Interlocking coupler was a 1954 development meant for freight service (Figure 7). Unlike the Type H, the Type F does not reduce slack, but it does provide better security in the case of draft gear failure or derailment. While all three modern types share the 11-inch knuckle height of the Type E, the knuckles are not interchangeable. The body casting of types H and F are also larger to accommodate the added features.

Figure 6: Type H Tightlock Coupler	Figure 7: Type F Interlocking Coupler

This variety of coupler sizes in the prototype suggests that the careful modeler needs to consider the dimensions of couplers as carefully as those of the wheels, trucks, and other operating details.

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Copyright © 1994-2003, by Bruce A. Metcalf, who would very much appreciate your comments. Updated 6 November 2005