Though it may at first appear counter-intuitive, second tracks actually make more sense in mountain terrain -- though they are much more expensive in that terrain.

Trains are going to be going much slower on the mountain segments, therefore occupying tracks for longer times.

A single heavy train struggling uphill can tie up a single main track for considerable time.

Trains are also going to be going slower downhill, due to curves and the need to keep the train under control. And, in mountain terrain, trains are much more likely to get into trouble, leading to situations where you want to run one train around another.

Also, in mountain terrain, you are more likely to need to move motive power around, such as sending manned helpers back downhill. (No this may not or may no longer apply on Donner, but Crawford Hill in Nebraska is a good example of a mountain grade with double track.)

In relatively flat territory, trains can go faster and therefore occupy track segments for a shorter time. Therefore it may make perfect sense for a single track in flat terrain to feed a double track in the mountains.

-- Ernest