How much "play" is in there in railroad car couplings?
The play in an American freight knuckle coupler is less than 1, maybe 1 if badly worn. One often overlooked subtlety is the use of play in getting a train started. Generally when a train stops it is the car brakes that do the bulk of the stopping. With the engines doing little stopping this leaves at least the front of a parked train in tension with all the play taken out of the couplers. If the locomotive starts moving forward from this position it has to overcome all the starting friction of every car in the train at once. So to start moving a heavily laden train the locomotive often begins by moving backwards first. Initially the loco only has to move itself until the play of the first coupler is consumed, maybe 1. Then the loco begins pushing the first car until the play in the cars rear coupler is gone, another 1. This may continue for a foot or more traveling backwards compressing the play in the couplers one after another, but only having to start one car rolling at a time. Now the loco can stop and begin to move forward, initially only moving itself for the initial 1 until the coupler to the first car connects. Then the car starts to go forward, and one by one the other cars follow suit according to the play in their couplers. The loco only has to start one car at a time from a dead standstill. Once a dozen or more cars are moving the effect loses importance as the inertia of the moving cars is sufficient to start additional cars without undue strain on the locomotive.