I almost always build a double track. In reality I go that because I’m somewhat lazy… Infrastructure building and manintenance costs are small compared to train maintenance costs therefore the more constant flow of the trains should be able to compensate the extra infrastructure costs.
If you are great with signalling and can balance passing loops so that the trains are evenly spaced out and in a constant flow (and this means adapting your passing loops with each train you add and probably works only if you have the same type of trains) you should be able to get slightly better results.
Again I don’t think that the difference would be so huge given the low infrastructure cost, but over 150 years you can probably avoid spending several millions in building infrastructure and infrastructure maintenance.