Newman Atkinson says:

older tyco and Bachmann engines could handle sharper curves 3 axle trucks usually will give you trouble on tight curves Small switch steam engines or diesel with 2 axle trucks can usually handle them. The reason beginner sets mounted the couplers on the trucks was to allow the coupler to follow the direction of the tracks better. The only problem with that is when backing up with much load the car would sometimes derail. Body mounted couplers are better but usually need shallower curves In your case shorter length cars can help your situation and still do good body mounted.
I travel to shows with a tight 18 inch radius loop track and an inner track of 15 inch curves. Shorter cars do well and couplers can be body mounted but longer crs need them to be mounted where the coupler has swing or are mounted to the truck. My engines are usually limited to 4 axle engines to make the curves. from Newman