Over time, old vehicles will cost more and more to keep running… that’s why it’s important to replace them eventually. Otherwise they’ll get to a point where they’re costing more to run than they’re earning.
Also, go back and check your bus routes every 20 years or so. As a town expands, your old route will end up needing to be re-planned to better cover the town. I often find it’s better to run two different smaller loops around a town rather than one huge one, so people don’t end up stuck on a 15-minute bus ride to the train station, and end up giving up instead of taking the train!
As a town grows to huge sizes, I presume you’ll need to add more than two routes to keep it well covered. I haven’t got that far yet though… I’m slowly wrestling my way through a game on Hard difficulty 😛