A couple of things:
Trains/convoys can be built into the terrain to either supply as specified object like a factory or field without regard to where the track/road ends; or the train/convoy can resupply objects within a specified radius of the track/road's end point. There is also a third possibility that the train/convoy is just there for "looks" and does not resupply anything. It could be that the server can not tell what is resupplied by trains/convoys that use radius, or the train/convoy destroyed was just there for "looks" and does not supply anything.
Trains, like convoys, are created every 10 minutes (600 seconds) and they take 10 minutes to go from their source to their destination. If the train is half way to its destination when destroyed, a new train will appear at the source in 5 minutes. Likewise, if the train was one minute from reaching its destination, the new train will be at the source in one minute. Therefore, as it is now, the reported down times is how long the new train will take to reach the spot it was destroyed.
It would be nice to get some insight on how it works for those of us that make terrains.