You need to reframe your questions to think this through properly.
As Tim M says, your most important constraint is the height difference between the bottom end and the top end of the railway. The gentlest ruling gradient you can have is constant. Any variation in gradient makes the ruling gradient steeper.
But anyone with a rudimentary knowledge of geography will appreciate the river valleys are usually concave, very steep at the top end and almost flat at the bottom end.
This mean that most railway lines in river valleys climb up the side of the valley at the bottom end, are high above the valley in the middle, and then return to river level at the top. This requires either expensive engineering work or sharp curves, in order to cross the tributary streams coming down the valley sides.
I'm more familiar with standard gauge than narrow gauge, but the principles are the same. A good expensive engineering work example is the Settle and Carlisle in the upper Eden Vale between Appleby and Ais Gill. A good sharp curves example is the climb from Bridge of Orchy to the County March on the West Highland.
As Tim M says steeper ruling gradients come with motive power costs. Steep gradients and sharp curves also affect journey times. The answers to your questions then become trade offs between initial construction costs, and ongoing costs and revenues.
