Knuckle Coupler

A Knuckle coupler.


An ICE's Scharfenberg coupler.

A coupler, or coupling, is a device for joining rail vehicles in a train. This is a major part on a locomotive, wagon, or multiple unit, third in importance after power-source and gauge. A coupler is part of larger draft gear assembly, that may include links, drawhooks, drawbars, yokes, etc.

Manual, Semi-Automatic, and Automatic CouplersEdit

There are 3 very general types of couplers used on railways worldwide:

  • Manual Couplers with manual only mechanical, pneumatic, and electric connections.
  • Semi-Automatic Couplers with automatic mechanical connection, but manual only pneumatic and electric connections.
  • Automatic Couplers with fully automatic mechanical, pneumatic, and electric connections.

Pin-and-link CouplersEdit

This archaic design, sometimes called pin-coupler, dates back to the very early days of steam traction. Today, this design is only used on historical narrow gauge railways. Operation is simple; a drawbar projecting away from the frame allows access to a vertical pin that holds a link in place. The link extends out the end of the drawbar. As two railcars come together, the opposite pin is raised, the link is placed inside the opposite drawbar, then the pin is lowered, thus joining the cars. Uncoupling is the reverse procedure. Although a very simple coupler, it was also very dangerous, causing thousands of injuries and eventually being outlawed and replaced after 1900.

Three-link CouplersEdit

The old three-link coupler has only recently started to be replaced on UK rolling stock. Preferred by railways for its simplicity and low cost, today it is typically used only on freight stock. Vehicles fitted with this design have drawhooks that connect to drawbars internally. Each drawhook has a permanent three-link coupler hanging vertically from the hook's base, and only one is used when joining occurs; when one coupler is lifted over the adjacent vehicle's drawhook. This system is flexible in the fact that a shunting pole can be used to lift the link over the drawhook, which is safer than being between the buffers. However, the downside to this system is that it is loosely coupled meaning that the considerable slack is taken up as the locomotive sets off — in other words the train sets off one car at a time — and when the locomotive stops there is a concertina effect whereby all the cars stop one after the other. This causes wear and tear on the rolling stock, especially brakes and buffers, thus only slow freight trains could be equipped with three-link couplers. An upside is less of the train's weight is on the locomotive while its starting up the train; a reason why it survived for so long. The type is compatible and interchangeable with screw couplers.

Instanter CouplersEdit

The instanter coupler was an improvement over the three-link coupler. Its basic design was similar, but the two links that were held by the drawhooks were themselves joined by a pear-shaped object with notches in its design. These notches allowed for two states: an open state and a closed state. A shunting pole was used to switch it between states; the open state kept cars joined but allowed uncoupling, while the closed state held cars together while running.

Screw CouplersEdit

The most common type used in Western Europe and the UK, it was a development of the three-link coupler. In essence this is a three-link coupler with a threaded rod replacing the center link. A small slide-handle was used to pull the cars together or push them apart for coupling or uncoupling respectively. This was quite a tricky task as it was hard and fiddly to achieve the operation, and it certainly could not be done via use of a shunting pole. This design was used on passenger fleets, or fast freight trains. This type is compatible and interchangeable with three-link couplers.

Janney CouplersEdit

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A complete Janney coupler assembly.

The Janney, also known as AAR couplers, is a semi-automatic design. Between 1893 and 1900, this design replaced the outlawed and dangerous Link-And-Pin coupler on North American railroads. Outside North America, the Janney coupler was slow to be adopted, due to it being incompatible with existing designs. In Britain, Australia, and New Zealand, the Janney is known as the "buckeye" or "swinghead", being used on some unit freight and passenger trains. The swinghead coupler can be pivoted down or to the side so a chain coupler can be used. Janney couplers have a hinged knuckle; at least one of the coupler knuckles must be open as 2 cars come together. As coupling occurs, the open knuckles close against the palm of the opposing coupler and clasp each other, then a pin automatically locks the knuckles together. To separate cars, the couplers are pushed together for slack, the locking pin is raised with a side lever, the knuckles release and the cars can be pulled apart. Janney couplers were a vast improvement over Link-And-Pin couplers, saving many lives and reducing costs.

For Rotary Gondola|Hopper wagons and HazMat Tank wagons, the AAR compatible subtype F, also known as InterLock couplers, allow rotary dumping and prevent decoupling or telescoping during a derailment.

For Passenger wagons, the AAR compatible subtype H, also known as Tightlock couplers, minimize slack and prevent decoupling or telescoping during a derailment.

Willison couplerEdit

The Willison, also known as the SA3, is a semi-automatic design, primarily used in Eastern Europe and Russia. The C-Akv version of the Willison is compatible with European chain couplers. Operation is simular to the Janney coupler, but a locking assembly is used instead of a pin.

Scharfenberg couplerEdit

This coupler is fully automatic, including pneumatic and electric connections. Developed in the 1930s, it is the most common fully automatic coupler for unit passenger trains worldwide. As the couplers come together, a spring releases the brake pipe cocks, and a rotating cover reveals contacts for electric connections. To uncouple, a pneumatically operated release is activated from the cab. A variant of the Scharfenberg coupler, the Shibata coupler, is the standard coupler used on passenger trains in Japan and Korea; the coupler was developed by Japanese Government Railways engineer Mamoru Shibata who gave the coupler its name.