Buffers and chain coupler#Center-buffer-and-chain(s)

File:Consett-tankwagon41 - buffer beam.jpg

A peculiarly British practice was the "loose-coupled" freight train, operated by the locomotive crew and a "guard" at the rear of the train, the successor to the brakesman of earlier times. That train type used three-link chain couplings for traction and side buffers to accept pushing forces but, since such trains were not fitted with an automatic through-train braking system, there were no pipes to connect between the vehicles. The last vehicle of the train was a heavily ballasted guard's van with its brakes controllable by a handwheel operated by the guard.

The slack between vehicles coupled in this way was very convenient when starting heavy trains with a relatively low-powered locomotive on the level or on a rising gradient. On the driver's command the guard would apply his brake as hard as possible. The driver would then gently reverse to close up the wagons on to their buffers. Then the locomotive would be driven ahead, picking up the load wagon by wagon, thus giving an easy start up the gradient. Wagons of that era did not have roller bearings and the grease-lubricated plain bearings exerted considerable resistance to motion, especially on a cold day, so starting wagon-by-wagon in this way allowed the locomotive to move off with low initial resistance.

The disadvantage of that convenience was that the guard could be badly thrown about as the train changed speed due to the inter-wagon gaps opening or closing. In the worst case, the jerks could break a coupling or cause a derailment. A skilled guard would observe or listen to his train and apply or release his brake to keep the last few couplings reasonably taut, acting as a shock-absorber. The same effect occurred when the route changed gradient. When going over a hill the rear of the train would catch up with the wagons held back by the locomotive, but the guard could minimise that. That method of train working was why the guard, just like the driver, was required to prove his route knowledge before being given charge of a heavy train. Loose-coupled trains travelled at low speeds and were phased out in the 1970s.{{cite web |url=http://www.wsr.org.uk/couplings.htm |title=WSR :: West Somerset Railway :: Couplings |access-date=12 November 2006 |archive-url=https://web.archive.org/web/20060926185507/http://www.wsr.org.uk/couplings.htm |archive-date=26 September 2006 }}

An improvement on the loose-coupled train is the "Instanter" coupling, in which the middle link of a three-link chain is specially triangular shaped, so that when lying "prone" it provides enough slack to make coupling possible, but when the middle link is rotated 90 degrees, the length of the chain is effectively shortened, reducing the amount of slack without the need to wind a screw. The closeness of the coupling allows the use of inter-vehicle pipes for train brakes. Three-link and Instanter couplings can be operated entirely from the side of the wagons, using a shunter's pole, which has a hook on the end, and is safer when shunting is being done. Similarly, the screw-adjustable coupler can be connected by a shunter's pole once it has been unscrewed. Ordinary three-link couplings have been superseded by instanter, screw or buck-eye couplers in UK freight trains today.

{{Main|Balance lever coupling}}

File:Uzd7742.jpg flat wagons, {{Track gauge|750mm|lk=on}}. Note the single buffer with a hook on the right side and a chain on the other]]

On some narrow-gauge lines in Europe, and on the Paris Metro, a simplified version of the loose-coupler is used, consisting of a single central buffer with a chain underneath. Sometimes there are two chains, one on each side of the coupler. The chain usually contains a screw-adjustable link to allow close coupling. These variants are also used elsewhere. On sharp curves, a single centre buffer is less likely to be subject to buffer-locking. The Eritrean Railway also uses a centre buffer and chain coupler.

File:Ueberpufferung.png

On sharp reverse curves, the buffers can become locked by slipping over, and on to, the back of each other. Although careful track design makes this a rare occurrence, a series of derailments at Stuttgart Central Station in 2012 were caused by buffer-locked wagons. Buffer-lock could occur on very sharp switches on rolling stock with the older, rounded buffers. Newer buffers are rectangular and are wider than they are tall. Buffers and chain couplers allow curves to have around {{cvt|150|m|ft|0}} radius, but sharp S-curves are not allowed. If it were not for the couplers, much sharper curves could be allowed, provided the train is not too long.{{Cite journal |last=Green |first=J. I. T. |date=6 November 1964 |title=Buffer Locking on Reverse Curves |url=https://trid.trb.org/view/11573 |journal=Railway Gazette |volume=120 |via=trid.trb.org }}

The width between the buffers tends to increase as the track gauge increases and decrease as the track gauge decreases, which means that if wagons are changed from one gauge to another, the buffers might no longer match. That is because the buffers are originally extensions of the frames, which are spaced according to the gauge of the track. As well, the height of the buffers is usually lower on narrow gauge railways, corresponding to the generally lower height of the rolling stock. Therefore, narrow gauge railways often use centre couplers without buffers.

However, in the case of Iberian broad gauge railways, the height and spacing of the buffers are the same as for standard gauge railways in Europe including Great Britain, in order to allow through-running of rolling stock by the use of bogie exchange.

Buffers and chain couplers tend to have the two buffers spaced according to the gauge, but especially in Europe this is modified to the standard gauge value to allow interrunning by means of bogie exchange.

Dimensions showing variation of spacing by gauge:

File:Mh eisenbahn schraubenkupplung.jpeg|Buffer and chain screw couplers

File:Togo Eisenbahn 086 1990-02.jpg|Buffer and twin chain German-heritage couplers in Togo

File:Railway tanker at Washford Station - geograph.org.uk - 943849.jpg|Buffer and chain three-link coupling on a tank wagon

File:Balancierhebelkupplung-außen.JPG|Balance lever in front of buffer support (balance lever coupling)

File:Balancierhebelkupplung-innen.JPG|Balance lever behind buffer support (balance lever coupling)

File:CH FO HGE44-105-2.JPG|Electric locomotive of the Furka Oberalp Bahn (today MGB ) with balance lever coupling

File:Puffer-02.jpg|Buffers and chain coupler on goods wagon. The chain hangs on the towing hook.

File:BB 1056 Cambodia0092.jpg|Center-buffer-and-chain used on the Royal Railway Cambodia BB 1056 diesel locomotive in Cambodia

• Gangway connection
• Railway coupling by country

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• {{Commons category-inline|Buffers and chain coupling}}

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Category:Couplers