There are many makers of knuckle couplers in HO scale. All are compatable with each other, although specialty couplers like scale or shelf knuckles may have some difficulty coupling or uncoupling automatically when coupled to a knuckle from a different brand.
The original is still one of the most recognized names in the hobby. Kadee® offers the most extensive line of couplers, including many special adaptions for converting older cars and locomotives, scale and shelf knuckles. Couplers are marketed by number. The No. 5® has been the defacto "standard" knuckle coupler in HO scale for generations. Kadee keeps an extensive conversion chart on their website and in cataloges.
All of Kadee's® couplers are made of metal and have a reputation for durability and reliability backed by decades of experience. Kadee also markets uncoupling ramps, spare springs, lubricants and gauges to make the most out of your couplers. A Kadee coupler gauge is a must have and will keep any coupler in line.
Since the expansion in the HO coupler market in 1997, McHenry™ has emerged as an innovator in the field. McHenry offers an ever-increasing line of knuckles, including scale and shelf varieties and some special adaptations. Most couplers are made of plastic but feature metal knuckle springs. Some early versions, including the standard No. 1 coupler, featured a plastic knuckle spring.
Accumate® by Accurail
Accurail, well-known for their HO scale freight car kits, markets their own line of couplers. Accumate® couplers work much differently than other brands. The couplers utilize a split shank arrangement whereby the entire coupler seperates to open. The centering springs, cast onto the end of the shanks, also close the knuckle.
Despite the differences, Accumates® will connect to other couplers and work with track magnets. Accurail also markets a "switchman" manual uncoupling tool.
Accurail has recently introduced a scale version with exceptional fidelity to prototype dimensions.
E-Z Mate® by Bachmann
Bachmann's E-Z Mate® couplers feature all-plastic construction with cast on centering springs and knuckle spring. Short, medium and long lengths are offered with over, under and center-set heads.
Intermountain markets a limited line of knuckle couplers. Intermountain's couplers are made of plastic and feature cast-on centering "wings." Most feature metal knuckle springs.
Proto 2000™ by Walthers
Proto 2000™, originally a high-end product line from Life-Like Trains is now manufactured by Walthers. Only one size is offered, similar in dimensions to others' standard couplers. They feature plastic knuckle springs and metal centering springs.