I noticed that killing each train car adds .7 minutes of downtime so I assume each train is 7 minutes.
Still.....from what I observed, HQ still comes up faster than the values of each resupply whether train or c47 or m3.
Since I once observed trains respawning a quarter mile down the tracks after I killed every car of one, I'm wondering if it's simply a timer and not the train itself that determines it's resupply value.