It's been a while but I'm pretty sure the actual way it works is:
- there is a set number of convoys in a game
- spawn a whole convoy whenever a convoy contains 0 trains
Therefore, the proper method of handling trains is to destroy all but one irrelevant train per convoy, and then astrotrains will cease to have any impact at all on the game.
I seem to remember the last time there was a discussion one of the wiki contributors had obtained good info and updated the wiki with it.