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 modify-able by ...

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 ...