Without railroads, the economy in real life would never have grown as big as it has. That's why I would suggest industrial advantages or synergies with districts. 


Railroads often border industrial areas so that goods can be transported quickly and easily from there. The current synergy is unfortunately very weak and not really worth it. In addition, you only need one station for the bonus, which does not necessarily have to be connected to another station. It is important that there are good and many connections between the stations, and thus also between the industrial districts. 


Idea:

I think the station should get an industry bonus, which should depend on these factors (industrial district adjacent; number of stations it is connected to, which may still depend on how much industry the connected stations produce). 

(Another idea would be that railroads leading to foreign countries generate money).