Track-side refuelling - Let trucks refuel trains.

As it is, when a train runs out of fuel, the only way to refuel it is with unity. No exception.
This also relies on you noticing that one of your trains is out of fuel to hit the button, ideally before your entire economy collapses.
Trucks assigned to fuel depots with the appropriate fuel should be able to (assuming it can pathfind to it) run some fuel over and do a track-side refuel.
This should only be possible when it's completely empty, of course, as refueling a moving train from a truck would not be reasonable. This is not Fast and Furious, after all.