Open TTD v.1.9.3
Please any native english speaker, read through my text and correct any miswording (could be a lot^^)
Enable Infrastructure Maintenance
Options (ingame) --> Settings --> Accounting
The maintenance costs grow overpropotionaly with the network size.
With this bigger companys are more burdened than smaller ones.
In late game short connections between industries or cities may not earn money or even cost you while in you finance reports is a surplus of income. With this setting ON the player has not only the network maintenance working against him, but also the inflation which increase the annual costs for the maintenance further. For further information look through "Economy" 
Infrastructure Window -Ingame-
Network Size Calculation
The costs depend on your actual tiles you use for rails and streets, and the number of intersections present on one tile. (fig.3)
This results in higher costs for complex networks in a small space vs. stretched out networks with more space between intersections.(fig.4)
Rail Intersection Example
One rail on one tile counts as 1 rail piece (1x1)
Two intersecting rails on one tile counts as 4 rail pieces (2x2)
Six intersecting rails on one tile counts as 36 rail pieces (6x6)
A straight rail devided by a street counts as 2 rail pieces. Like the street.
Rail Summary The game squares the intersections on one tile and counts the result as rail pieces.
Street intersections are counted as 4. For each tile site with a road piece (fig.5)
Minimum tunnel lenght = 2
Street tunnel piece count = 8 (10 tiles = 80)
Railway tunnel piece count = 4 (10 tiles = 40)
The same piece count (fig.6) goes for bridges
Overpropotional cost grows
Besides the intersection multiplicator the game has an algorythm which increases the costs for your network overpropotionaly. (If someone can find the algorithm, please insert it here)
Here is an example list for the cost grows, with an roughly factor which shows the cost increase from one row to the next. (Used Maglev rails)
Summary cost increase
You can say with every rail piece amount duplication, the costs inrease around 280%
(exp. 936€ for 40 pieces x 2.8 = costs for 80 pieces). (fig.7)(fig.8)
Unfortunaly to this date I know no workaround or economical solution/trick. One may think that the game can be tricked with rails over a tunnel or rails under a bridge, but no. These setups other than the saved space do not reduce the maintenance for the tile. Even though no roads or track are intercepting.
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