please allow me to ride along on your suggestion, and possibly extend it. In the fuel tab of the train properties, enable a conditional for "seek fuel", maybe settable as to percentage remaining. in train stops, enable a condition for "I am a fuel stop". This condition could be ...

Just like Exit-conditions that are in the game, Pre-conditions would be nice, too. They'd work just like exit ones, except the train checks them before it makes a route to the next station. This could be something like: Fuel < 100 - The train won't stop at this station unless it's fuel is below 100....