How Does A Railroad Coupler Work?

Railway couplings are the mechanisms that connect locomotives and rolling stock in order to form a train. These devices have evolved throughout the existence of the railway, thus, many different types of couplings have been developed.

How does a railroad coupler work? Railroad couplers work through a pair of steel knuckles that automatically lock into place. In the UK and continental Europe, buffer and chain couplers are used where a worker must make the connection manually. Fully automatic couplers are also commonplace on high speed trains, as well as multiple units and mass transit vehicles.

There are various types of couplings in use throughout the world, many of which have evolved from a time of where couplings lacked standardization, proved dangerous for the railroad employees, and were constant maintenance headaches. Through over a century of development, couplings are mostly standardized on various rail systems throughout the world. Various types of the most prominent variations of couplers will be highlighted in this article, along with a brief history of their development.

Type H knuckle couplers .Ed Gately Photo


Since the genesis of the railway, various systems have been implemented worldwide to couple locomotives and rolling stock. The earliest designs proved effective, however, they proved treacherous for crews coupling the cars. Some of the earliest shortcomings of these primitive coupler systems were faced by the rail workers, such as brakemen, as they were the individuals risking injury to successfully couple cars together. In addition to the safety risks to train crews, many passengers experienced an uncomfortable ride due to the high slack action, and the cars bumping each other.

In North America, the most common type of coupler is the semi-automatic knuckle coupler, which automatically locks in place if one knuckle is in the open position. However, the brakemen must still climb between the cars to connect the air hoses. Automatic couplers are also utilized on various passenger and other transit systems in North America.

In the UK and continental Europe, the buffer and chain coupling is commonplace. When connecting rolling stock, the buffers will touch, and a rail worker will connect the screw coupling to a hook on the opposite wagon, carriage, or locomotive. This is perhaps one of the oldest methods of coupling, as its origins date to the early 1830s, first being utilized on Stephenson’s Planet locomotive.

Link and Pin Coupling

The earliest coupling systems in the U.S. consisted of link and pin couplers. These types of couplers consisted of a link, with a “U” shaped hook on the end. When coupling cars, a rail worker would have to place the pin into the hole in the top of the link, securing the “U” hook into the coupler pocket.

This was a tedious task, and posed an extreme risk for the railroad workers, as they were oftentimes injured during this process. Unfortunately, many rail workers lost limbs from facilitating the coupling of cars, some injuries even proving fatal. Additionally, these couplers lacked proper standardization, thus, many cars were either difficult or near impossible to connect. This lack of proper safeguards for railroad employees, as well as the lack of standardization between couplers led many to develop new universal designs.

In the late 19th century, innovations in coupler development were paramount. However, one individual, Eli H. Janney, developed a type of semi-automatic coupling system, in which the coupler locked in place automatically, however, the brakeman still had to climb between the cars and connect the air hoses for the brakes.

Domingo Kauak

Janney Coupler

Developed by inventor Eli H. Janney in 1873, the Janney, or knuckle coupler, revolutionized the North American railroad industry by creating a universal coupler, compatible with almost every railcar on the system. However, the couplers continued to lack standardization until the Master Car Builders (MCB), developed a standard for the knuckle coupler, allowing all rolling-stock utilizing the coupler to be compatible with each other.

Although all rolling stock utilizing the MCB couplers were compatible, various components of the coupler, such as the dimensions of the knuckle, differed, making maintenance a headache. The early 20th century gave way to a universal design, where all parts are interchangeable, and each company that submitted a design would have the necessary means to access the other’s patents.

Two companies submitted their designs to the MCB Coupler Committee, which were American Steel Foundries, and National Malleable Castings. Upon these submissions, the designs were further developed by the committee, and designated the Type “D” coupler, which was introduced into service in 1916. Further development of the type D coupler came in the form of the type E, which is still the most common coupler utilized on freight cars in the modern day. Interestingly, according to the Central Pacific RR Museum, with the exception of the utilization of higher grade steel, little has changed in the design of the type E coupler since its inception in 1930.

The type F, or “shelf coupler”, is commonly used with rolling stock transporting hazardous materials, such as tank cars hauling crude oil. Further developments of the knuckle coupler resulted in the Type H coupler, which is mandated for use on passenger equipment. The type H couplers are high tensile, and are equipped with built-in anti-climbers which prevent cars from uncoupling in the event of a derailment.

Buffer and chain couplings on a Class 58 locomotive. Steve Jones photo

Buffer and Chain

Originating in the United Kingdom in the 1830s, the buffer and chain coupling system has become the standard in the UK, as well as in continental Europe. This type of coupling was initially utilized on Stephenson’s Planet locomotive in the early 1830s, which soon standardized the coupling system in the UK. These are considered the standard coupling of the International Union of Railways (UIC).

These types of couplers are made up of three components, two buffers, a chain or screw, and a hook. Today, this type of coupling is equipped with a turnbuckle screw, which tightens the gap between the two couplers, therefore, reducing slack action.

Coupling carriages and wagons utilizing this type of system operates as follows: two pieces of rolling stock are brought together until their buffers touch, a worker then climbs between the two wagons and connects the chain or screw on the adjacent hook, then tightens the turnbuckle screw to lessen the slack, finally, the worker connects the air hoses for the braking system.

The buffer and chain coupling has significant advantages and disadvantages to the semi- automatic knuckle couplers. Some advantages is less prominent slack, especially on curves, and are less prone to breakage compared to its semi-automatic counterparts. However, the real disadvantage of the buffer and chain coupling system is the weight that the system can handle. This is especially prominent with freight trains, as they can only handle a maximum of 4,000 tons. However, in the UK, knuckle couplers are oftentimes utilized on certain trains to allow for heavier loads.

Fully Automatic Coupler on a CTA car. Jonathan Lee Photo

Multi Function (Automatic) Couplers (N-Type/Scharfenberg Coupler/Dellner Coupler)

The multi-function automatic coupler is one of the latest developments of coupler technology. The multi-function coupler functions without the need for human intervention, as both the coupling mechanism and the air hoses connect automatically. These are typically utilized in various transit vehicles, such as subway cars and light rail vehicles.

There are various types of these kinds of couplers in use throughout the world, as some variants utilize a knuckle coupler, and have electrical equipment pickups below. In addition to their utilization on mass transit trains, they are utilized on heavy rail as well. High speed trains oftentimes utilize these types of couplers, as well as diesel and electric multiple-units.



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