The train journeys up to Oban, Mallaig and Kyle of Lochalah are stunning, and Arran is easily reached via the Ardrossan Harbour train. You go through spots you can't get to by road, like Rannoch moor, and can enjoy the scenery more.
But... The services are infrequent, and you're tired to limited areas and a very outdoorsy existence while you're there. Car is much easier and more flexible, even if the train has charms of its own.