Title says it all. I don't use trucks only for transporting coal/fuel so I will see my sad little tractor turning its front wheels and not going anywhere, just pouting. It would be nice to know if they are out of fuel without having to travel to each one, and I have two thoughts on the matter.
1. Make the tractor location on the upper direction heading flash a bit once it is out.
2. A harder way to do it would be to make the tractors smart; the tractor records average fuel use between stations and determines if it can make the next trip. If it can't, it sits at the station.