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 proceed freely 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.

Slack action creates a danger of accident, particularly as the length of freight trains get longer. While accidents from slack action do occur in the operation of passenger trains, it does not appear that accidents occurring on passenger trains, whatever their length, are of sufficient severity to cause serious injury or damage.

Modern couplers

Coupling (railway)

A coupling is a mechanism for connecting rolling stock in a train. The design of the coupler is standard, and is almost as important as the railway gauge, since flexibility and convenience are maximised if all rolling stock can be coupled together.The equipment that connects the couplings to the...

are designed to reduce slack action over older technology, such as link and pin connections.