I wondered why my 1780 network was suddenly profitable after posting massive losses. Then I found the reason was a month with positive construction costs, although I had spent money to extend my network during this month. Apparently demolishing city buildings adds ~1/10 of their value to your money. Not sure if that's a bug or financial wizardry I don't understand...
In fact, costs for building roads are added to my income, too.
Thank you for the reports: I think that I have now fixed this.