Bingo. They programmed the maintenance of trains, without considering what would happen if someone went against the intended design of the game and used a train of buses across the map. But trains are not so expensive that it is hard to use them. In my games they are usually the primary money earners, because I use them and don’t exploit an unintended consequence of the game’s design.
No, they didn’t. I don’t use a “train of buses” (whatever that means) and trains full of people and goods constantly lose money. I have no intercity buses at all. Those two scenarios aren’t remotely related.