Even where the track gauge itself is the same, differing standards for wheel and flange dimensions (which in turn influence certain crucial dimensions of switch

Even where the track gauge itself is the same, differing standards for wheel and flange dimensions (which in turn influence certain crucial dimensions of switches and crossings) still have the potential to throw a spanner in the works and prevent full cross-compatibility. If the differences aren't too large, some sort of compromise wheel set might be possible – if that's not the case, then you'd still need to swap the wheels.