When trucks run low on fuel mid-job, they... just keep going, even once fuel hits zero, even with "truck without fuel stops working" set, perform their delivery, then keep driving on no fuel in order to reach the fuel station (though with the "truck without fuel stops working" setting, that's the only destination they can then drive to sans fuel).

On the other hand, when trains run out of fuel... there is only one way to fix that, and that's Unity refuel. They stop moving, and cannot move towards the fuel station. They tend to have much larger fuel reserves, though. Thus, two train-fuel-related suggestions:

1. Tenders. On Lines, we can order a train to stop at two fuel stations on route to pickup, then two more on route to delivery, if we so desire. On Networks, trains will not seek fuel anywhere between leaving a waiting bay and leaving the delivery stop, 2 full journeys away. Auxiliary Tenders, something that exists in real life to extend the already-long range of diesel locomotives, would be a suitable solution to this, ensuring that any train can carry enough fuel for a cross-map job, not just steam- and turbine-driven, the only two tender types currently in the game, and electric, which doesn't carry its fuel with it to begin with.
2. The first warning we get that a Network's fueling threshold is set too low is a train runs out of fuel and the network straight-up stops working. I'd like to have a "train min fuel" readout on the network details, to show the lowest amount of fuel accessible to the locomotives of each train type, sorted by fuel type or maybe consist, over the last X period- perhaps as a statistics-style graph? Notably, this should include tenders- but unlike the fuel threshold that considers any car (causing a 30% threshold to fuel any train whose tender has gone below 30%, even if the loco itself is still 100%, I find this behavior desirable), this readout should be for total fuel available to the train, so a tender running out while the loco still has fuel won't get it to show 0%, merely the 30 or 40% of total capacity that the fully-fueled locomotive represents on its own. We can then use these readouts (or graphs) to judge whether or not the network is at risk of running a train out of fuel, and adjust the fueling threshold- and/or train consists- accordingly.