I tried to figure out how to travel from Prestwick Airport to Dumfries by train...what a nightmare! I rented a car
Should be straightforward unless you are arriving at Prestwick late at night. Take a Carlisle or Newcastle train direct from Prestwick Airport to Dumfries. Or take a Kilmarnock or Glasgow via Kilmarnock train to Kilmarnock and change trains at Kilmarnock.