Specifically, "train may not depart this station unless it has at least X fuel". Why?
Because more often than not, my first warning that my train fuel station has run out of fuel... is a train running out of fuel, which becomes a forced Unity spend, and uncertainty over exactly how many trains didn't get the fuel they wanted. This is even if I configure the feeding storages with alerts; a single T2 steam locomotive tender can fit two fuel modules worth of fuel (144 tender space, 75 max in the fuel module), so a single train can empty the last of the storage- triggering the alert- and still leave with insufficient fuel.
This applies to both lines and networks- Line stations running out of fuel will simply dispatch the train with little or no fuel, and it'll continue on its way despite still desperately needing fuel. Like this one:

(You can see the upgrade designed to solve the station exhaustion problem...)
Network trains, if I remember a previous world correctly (behavior may have changed in the versions since), run in continuous circles around the fuel station until they run out of fuel- not even attempting the work they still have to do! That does, at least, tend to mean the fuel-out trains have a predictable location, and I don't have to worry about future fuel-outs once I restore the station...
So it'd be nice if I could tell the station (or the Line) that the train isn't allowed to depart unless it has at least X amount of fuel. It could- and absolutely should- throw an alert whenever a train is waiting for that condition (and not actively fueling), though, as such would indicate a fuel shortage.