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In railroading, slack action is the amount of free movement of one car before it transmits its motion to an adjoining coupled car. This free movement results from the fact that in railroad practice cars are loosely coupled, and the coupling is often combined with a shock-absorbing device, a "draft gear," which, under stress, substantially increases the free movement as the train is started or stopped. Loose coupling is necessary to enable the train to bend around curves and is an aid in starting heavy trains, since the application of the locomotive power to the train operates on each car in the train successively, and the power is thus utilized to start only one car at a time.
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Risk
Slack action creates a danger of accident as the length of a train increases. This is particularly the case when a train stops in a siding and is more or less compressed. Then the driver will release the brake while standing, while the locomotive direct brake applied and the train will stretch backwards and will possibly foul the switch/points behind the train. There have been very serious incidents with this effect. Slack action also can lead to severe longitudinal forces in a train that can result in a divided train. While accidents from slack action do occur in the operation of passenger trains, they are not of sufficient severity to cause serious injury or damage, regardless of train length. The draft gear of modern couplers is designed to reduce slack action over older technology, such as link and pin connections.
United Kingdom
The UK formerly used three link couplings which allowed a large amount of slack, actually these had been replaced by buffers and chain couplers . Where the couplings are tight by buffers and shortened by a turnbuckle, other railways decided to change it replaced by automatic couplings, such as the Scharfenberg coupler and the Janney Coupler.