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Knuckle Couplers



The standard knuckle coupler has evolved over the years, but principles and compatability remain. This knuckle can be found on the pilot of PRR No. 7002.

©2010 Ryan C Kunkle, licensed to About.com, Inc.

Knuckle couplers are available in every scale and an ever-increasing variety of styles. Most mimmic the look and operations of prototype couplers. Hobby standards insure compatability between most brands.

Prototype History

Knuckle couplers date to the 1870s in North America. Eli Janney is credited with the patent for what ultmately became the standard knuckle design in 1873. Janney's coupler was only one of many patented couplers competing for a hold on the American market.

Prior to knuckle couplers, train cars were hooked together through a simple coupling known as link and pin. These link and pin coupers were simple in design but deadly in practice. Unknown thousands of brakemen lost fingers, hands, even their lives standing between oncoming train cars waiting to drop the pin at just the right moment.

Link and pin couplers were as inefficient as they were deadly. The strength of these links limited the size of rolling stock and trains. They made switching cars time consuming and difficult. A better solution was needed.

The challenge for railroads nationwide was to agree on a successor. Every railroad had to adopt the same design in order for cars to interchange between lines. It was an expensive and logistical challenge. The United States Congress finally settled the argumet in 1893 with the passage of the Railway Safety Appliance Act. Among other things, the law demanded the use of knuckle couplers on all interchanged equipment.

Since then, the knuckle coupler has undergone several revisions, gradually growing larger and stronger. Key dimensions however have never changed so couplers from a 100 year old freight car could still lock arms with a modern locomotive.

The World's Strongest Handshake

The story goes that the original idea for the knuckle came from shaking hands. From above, a pair of knuckle couplers do resemble human hands. How then do you release the grip?

The pulling face of the coupler (the knuckle) is hinged. A pin at the rear of the coupler drops into the knuckle when closed to hold it tight. To pull the pin, the brakeman only needs to lift a bar, the cut lever, on the end of the car. The coupler will lock automatically when it is closed, including when contacting another coupler.

Model Knuckles

The tiny details and functions of knuckle couplers have been a challenge to modelers for generations. In smaller scales, the company to set the standard for decades was Kadee. Kadee's 1947 patented design became so universally accepted that it's name remains synonomous with knuckle couplers to this day. In addition to designing a working coupler, Kadee developed a strong line of couplers that could be used to convert just about any piece of model rolling stock. These included different length shenks, offset heads, odd-sized mounting holes, and a complimentary line of boxes, washers and more to make converting anything possible. The line also expanded into other scales.

Made entirely of metal, most Kadee couplers use a pair of springs to keep everything centered. One in the coupler itself keeps the knuckle closed. A second, in the mounting box, keeps the coupler centered to facilitate coupling.

Most ingenious of all, Kadee's coupler used a curved wand below the knuckle (designed to resemble an air hose) to react to magnets between the rails for automatic uncoupling. Modelers finally had a coupler that was sturdy, reliable and automatic.

Since Kadee's first patent expired in 1997, numerous manufactures have come forward with compatable designs. Although all of the brands will interact, coupling and uncoupling different brands is not always as smooth as using two of the same.

HO Coupler Guide


The magnetic uncoupling function of most knuckle couplers is a true asset in every scale. Magnets installed between or below the rails pull the wands apart, opening the knuckle. By backing up slightly, then pushing forward, the couplers will remain open but in a "delayed" position. The car can then be pushed off of the magnet to a final spot. Back up and come forward again off of the magnet and the couplers will engage automatically.

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