What are the length of the "branchline" cars? The ConCor branchline passenger cars are around 60' IIRC. I have no trouble getting them around 18-in. radius curves. Now, as was suggested by pdqtimetrialer, it may be necessary to trim some of the details from the underside of the cars IF there is any interference in swing. Likewise, I have no difficulty running Athearn 72-ft cars around 18" radius curves. I have typically used standard-length Kaydee couplers in the truck-mounted gear boxes. The only caveat is that you do have to trim diaphragms somewhat so they don't interfere on the curves. I use Walthers diaphragms but cut one or 1-1/2 "folds" off before cementing the strike plate. I also bend the vertical part of the plates at a slight angle back toward the end of the car and compress the folds slightly. It is true that when running on the tangent (straight) track, there will be a gap between the strike plates, but that is the compromise you must accept in some things.
I do have some difficulty with running 85' passenger cars to run reliably around tighter than 22" radius, and tend to intermix 85' cars with 72' cars.