Basically, the distance between coupled cars is basically the length of two coupler heads plus the amount of draft gear travel on each car and the thickness of their striker plate castings.
Draft gear spring travel before the time of super-cushioning and extended draft gears from the late 50's onward, was about 3 1/2 inches or so per car. About 6 inches of travel for cars with Duryea cushion underframes, introduced in the 1930's. They were no longer allowed for interchange by the 1960's as not all roads had the equipment needed and expertise for repairing Duryea underframes,
Often omitted on many models is the striker plate casting, through which the coupler shank passes under the end sill to the draft gear. On many models I see use the little flange on the end of the Kadee coupler lid for that, when it should be at least 1/16 (3"scale) out from the end sill or car end sheet. I use pieces of 1/16" square styrene for that, cutting the ends on a taper to match the Kadee lid.
The Weaver B&O wagon top box car needed a bit more detail than that. The couplers extend farther out, following Duryea design. The detail helps fill in the gap under the end sill, and models the cast steel end sill bracket holding the end of the center sill and coupler mount.
The Duryea does not have draft gear a such. The car body is isolated from center sill. On impact, or slack run-in and run-out the car body can shift back and forth up to 6" on the center sill, being held in place by spring nests on the inner face of the bolsters. The space between this casting's face and the back of the coupler head is 9" on the prototype. Notice the crack in the end sill bracket/carrier casting, caused by impacts over along time.