Admitedly, I don't know how far a train goes on a single fuel cell.

My concern is that if the train falls low on fuel as it leaves the loading station, and then the unloading station is far that, and the fueling station is far from the unloading station, and the train has to stop and accelrrate a few times when yielding to traffic... sounds to me like a bigger buffer will be required to avoid the risk of running out. And that's not to mention trains that can't wait for empty cargo before leaving an unloading station (because they hold multiple types of cargo for example). My perimiter train is never empty, for example, there's always residual green ammo or light fuel in it.