The Kyle line between Inverness and Kyle of Lochalsh is reputed to be the most scenic train journey in Britain. I dare say, it's difficult to refute. Just look at this. Really.
My advice for the best pictures, assuming the weather is good, would be to travel in the morning on the right-side of the train from Inverness to Kyle of Lochalsh. The right-side has the better views, particularly once it hits coastline towards the end of the journey, and the sun is out of your face.