A simple mod that will cause your trains to go to a fuel station automatically when they get low on fuel.
This is done by naming train stations a specific name (default is "Global_FuelStation", which can be changed in the map settings). When a train gets low on fuel (based on the map setting "Min Fuel Value"), it will add the train station to its schedule (as a temporary stop) and go to it. It will stay as long as needed (leaves based on inactivity).
The trains only check for low fuel when the train stops at a train station (mostly for performance reasons, but also just for simplicity). If the train needs fuel, it will go to the refuel station as its next stop.
This mod was mostly uploaded so that when I play with friends it is easy for them to download and update when needed.