Sorry for bad english. That’s the question. When using more than one train in a single line, ¿Should I build a single track and add passing loops, or is more profitable building two parallel tracks (One for each direction)?
I know how to use signals correctly, even for giving more priority to a direction when using passing loops, so that’s not a problem at all.
Problems I noticed with the methods mencioned above:
Single tracks require so much passing loops for a constant flow of more than 2 trains. That ends like 2 tracks but with some spaces between.
Double tracks cost more money and also are expensive to maintain.
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.
I usually build double tracks on my main inter-city lines initially as it to saves the hassle of having to upgrade them later. I find that with single track lines its almost always necessary to upgrade later as traffic grows. (I always start in 1850 on a large map.)
For smaller dedicated freight lines, or short branch lines, a single track generally adequate. Once they are running satisfactorily its not necessary to increase the frequency as traffic grows, just make the trains longer.