The consumption depends on the number of active players in a city and the progress of the city compared to the time played.
As we have a lot of players on this server and the first levels were fast, the consumption is quite high now. There is a maximum of 30% that has been reached in some cities. The consumption will slow down a bit now for level 4 cities. Some cities managed it to become level 5, though. Keep pushing!
With your permission, Bruno, it's better to explain everyone this concept: 30% of consumption means (5.700*0.7)=4.020 but 5.700 is required level up (4.020) tons increased of 42%.
It's really too much for our actual resources (Rhinos). I'm sure we all can do better with more tractive force trains, later.