You mean, if I own 1000 track tiles, I pay more than twice than if I own 500? Is this feature mandatory?
Yes. Otherwise the only thing that this patch would do is reduce the profit margin by a small bit.
Given that so far, maintenance costs don´t exist at all, I think it would be more than a bit (of course depending on how big the constants are...). I mean, there is for example one train set, I think it is called American (or similar) that has really high maintenance costs for the engines. Linear costs with respect to the number of engines you have. And my experience is that money is actually a factor in such a game.
Where's the sense in (warning: simplification!) double tracks == double profit == double maintenance cost?
So far, we have double tracks = double profit = no maintenance costs.
And why do in reality (linear costs) railway companies try to remove any junction they don´t actually need? Because their, linear, costs actually count.
I think linear costs would do their job, if the constants are chosen sensefully. This may for example include that junctions are expensive. If although there are such costs, the players earn huge amounts of money, I would rather have a look on the income rates.
Furthermore: My playing style involves building up a big railway network (but e.g. single track lines as long as they suffice) while "stopping" time. E.g. at the end of every year 1934, I go back to the beginning of 1934. Would with non-linear costs be building up a map-wide railway network with the 1934 economic settings work?
It's only a challenge if it's double tracks == double profit == more-than-double costs, which means you really have to think about whether you really need to double-track everything or really need those extra five junction tiles. And just to be exact, it's counting track bits and not tiles with track on. And junction tiles with overlapping track bits are penalized extra, don't fill all your map with those XXX station entrances.
Given that making a railway company profitable seems to be a challenge in reality, I think using realistic economic settings should do their job to make the game challenging. And non-linear costs don´t have any counterpart in reality IMHO.