David says:

Will you be doing a lot of shunting, or running trains as fixed rakes.
I prefer the Kaydee couplers because I like to make and break up trains and push troublesome trucks around. The uncoupling capacity of Kaydees makes this a dream, however some guys with mixed couplers have short, fixed rakes with Rapidos between them, and the truck or wagon at each end have both. This allows the train to be broken up easily, but in fixed blocks rather than individual wagons.
To retrofit Kaydees is not difficult, but make sure you have a fixing gauge to ensure all is square.

A tip to ensure the couple shank is kept at the horizontal (for optimal operation) is to put a small piece of styrene (plastic) at the inner end of the coupler pocket when attaching it to the wagon. This tips the front slightly upward counteracting the coupler head’s tendency to droop.