Complex sentence: 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.
Split into simpler sentences: In H0, the Americans standardized on horn-hook, or X2F couplers. Working knuckle couplers are a closer approximation to the "automatic" couplers used on the prototype there and elsewhere.