some longer passenger cars, i believe bachman is one make, have a body mount coupler where the whole coupler box pivots instead of just the coupler itself. those would work better on sharp curves. but you'd want to avoid having short and long cars coupled together. the ends of longer cars can swing out and pull the car coupled to it off the track.
that said with 18: curves i would limit myself to small steamers, 4 axle diesels, and cars of 50 foot or less.
that said with 18: curves i would limit myself to small steamers, 4 axle diesels, and cars of 50 foot or less.