The Bachmann Freight Cars are made to a Ohio River and Western Prototype Box Car #605 plan. The OR&W box cars are a bit high for most narrow gauge equipment. I replaced the stock couplers with McHenry Center Shank spring couplers this made the couplers too high. The following adjusts both problems. I now use the Accumate Standard HO because it is the correct size for most narrow gauge lines, the Kadee #5 is too small and the On3 coupler had limited narrow gauge applications. The pictures show McHenry center shanks.
The stock underslung coupler, prototype coupler shanks are centered. Why was this the choice?
Both cars with the center shank couplers, the one on the left lowered to align the coupler and the right car at stock height. The modified car will still negotiate an 18" radius curve.
The insertion of the center shank coupler makes the coupler height too high. I use the Bachmann Mogul tender coupler as my height gauge, it is easier to find than my Kadee gauge.
The stock truck has a lug on the top equal to the over height of the coupler. Remove the lug of the truck so the bolster is flat across. I used a motor tool with a 3/16" dia cylindrical burr to remove the lug.
The truck modification lowers the car and puts the coupler at the right height.
Accumate Standard Size HO Coupler offers a klunkier HO coupler which makes them perfect for On30. They don't have that "pencil neck geek" coupler shank that makes the head look like it is suspended in space.
(Click picture to enlarge)