Railway Modelling

In addition to different scales, there are also different types of couplers for connecting cars, which are not compatible with each other.
In H0, the Americans standardized on horn-hook, or X2F couplers, though these have largely given way to working knuckle couplers (Kadee) which are a close approximation to the "automatic" couplers used on the prototype there and elsewhere. Also in H0, the European manufacturers have standardized, but on a coupler mount, not a coupler: many varieties of coupler can be plugged in (and out) of the NEM coupler box. None of the popular couplers has any resemblance to the prototype three-link chains generally used on the continent.
For British modellers, whose most popular scale is 00, the normal coupler is a tension-lock coupler, which, again has no pretense of replicating the usual prototype three-link chain couplers. Bachmann and more recently Hornby have begun to offer models fitted with NEM coupler pockets. This theoretically enables modellers of British railways to substitute any other NEM362 coupler, though many Bachmann models place the coupler pocket at the wrong height. A fairly common alternative is to use represantations of chain couplings as found on the prototype, though these require large radius curves to be used to avoid derailments.
Other scales have similar ranges of non-compatible couplers available.