For added realism and even smoother operation through curves, borrow two techniques from the prototype; easements and super elevation. An object, espescially one as heavy as a freight train, moving is a straight line wants to keep moving in a straight line. These transitions help trains combat the forces of physics as they enter a curve.
Model trains may not have the mass of the prototype, but our curves are also generally much sharper. Even toy trains work better when they follow prototype standards. These may seem like complicated engineering projects, but some simple tricks will have you hitting those curves like a pro.