On my passenger cars, I use Kadee no. 5 couplers in their own boxes mounted directly to the carbody. As long as your curves are at least 30″ in radius and your turnouts are no. 6 or longer, you can take this simple and direct approach. Forget about mounting couplers on arms connected to the trucks or with long shanks or special boxes for extra swing. This is a lesson I learned from Chuck Hitchcock’s old Santa Fe Argentine Division layout, one of the finest examples of passenger train operation I’ve ever seen.
Chuck also mounted the coupler farther back from the end of the car than you normally see, so the cars couple closer together and the passenger train looks like a tighter unit. On heavyweight cars I follow Chuck’s recommendation and drill holes for the mounting screws 3/8″ in from the surface where the diaphragm will be mounted. This results in a coupled distance of about 16″, very close to scale.
Lightweight cars have square ends without the taper usually found on heavyweights, so this coupling distance may allow the corners of lightweight cars to touch on curves. When I saw that this was happening on my minimum radius, I decreased the coupler setback to 5/16″. This makes the coupled distance between lightweights about 20″.
To locate couplers consistently, use a simple marking gauge of brass strip bent 90 degrees, with a hole drilled 3/8″ or 5/16″ from the inside of the bend. I have two gauges, one for each setback.
You can use either a 2-56 screw through the middle of the coupler box or two 00-80 screws in the side ears. For brass cars I choose whichever screws will let the new holes miss any already drilled in the mounting pad.
From The Model Railroader’s Guide to Passenger Equipment & Operation (Kalmbach Books, 2006).
Body-mounted Kadee no. 5 couplers make for simple installations and operate reliably. Deep setbacks made consistent by rudimentary brass gauges provide close coupling.