Why do some couplings have different arm heights and lengths?

How far couplings protrude from their host wagon, loco or coach is something only kit builders worried about before NEM couplings came along. So why do we now have short, medium or long arm choices?

The truth is, the extent to which couplings protrude has always varied slightly across manufacturers, but it was always assumed that a ready-to-run Hornby wagon would always be coupled to a Hornby loco, a Bachmann wagon to a Bachmann loco and so on, thus, the slight differences in coupling heights and lengths was rarely talked about. But with the introduction of NEM couplings came the introduction of choice and interchangeability. To cope with this, a selection of couplings featuring common pockets but with arms varying in height and length came to market.

The height difference is dealt with by a small step up or down in the shank of the coupling. The difference in height allows a mixture of rolling stock types to be coupled at the same height reducing the risk of a derailment.

As for the variations in shank length, the main reason for this is to facilitate close coupling. Close coupling is when two coaches or wagons are coupled with a smaller gap between them to better represent the appearance of the prototype. However, that also makes them more likely to foul one another on tight radius curves. Having shanks available in three lengths allows modellers to find the best compromise between realism and practicality according to their track layout and rolling stock operated.