At first, and most of case, it’s certain omnibus wins.
When you get bigger city, you will get more than one station in the same city, then you have choice to make express between cities, it works well to.
Third, I made some loop lines between big close cities, it doesn’t earn well but helps on your traffic.
Last, make a omnibus loop at the end of express line in big cities which has some station is a super solution: you get better traffic, and you reduced the platform occupying time since there’s no train rebond.
