I've seen many photo's of fully loaded coal or other very long heavy trains, and wondered how much strength the knuckles in the engines/cars have to have to withstand the pulling load. What takes the most pressure, the knuckle, pin, drawbar, etc. How is their strength measured ?
The knuckle pin is simply used for a pivot point-- it does not take the pulling load. That load is taken by the cast-in contours of the inner side of the knuckle, against the coupler body inner face. It's easier to see in person (or in a diagram) than to explain!! There are actually PLASTIC knuckle pins available--they certainly don't take the full pulling load!!!
The castings are rated by tensile strength, and there is a numerical code cast into the coupler body and knuckle that signifies the strength. It's so that low-strength parts won't get mixed in. Many of the older (low-strength) components are now prohibited in interchange service.
For example, the E50 knuckle is now obsolete, replaced by E50HT--the HT signifiying "high tensile" strength steel.
There's a lot to this; I recommend the Railway Educational Bureau--Simmons Boardman publication about couplers for more (much more) detail.
