A bit offtopic question:
What's the point of astro trains? What are they supposed to be (analogy with some SF show/book/film etc)?
Well, I don't have a particular inspiration from an outside reference like that for them -- though obviously the Transformers character by the same name was something of an inspiration for the name. Thematically, the idea is that these are really heavily armored couriers, which have some weapons and thus damage stuff along their path. Thematically speaking, that's about it: it fits with the whole "the AI is mostly just doing it's own thing, which is a bit unfathomable to the humans, and it deals with you when it notices you" aspect (more on that
here).
There were a number of "points" to them, in more practical gameplay/ game design terms, though:
1. They provide another layer of complexity to the AI's activities. During alpha testing, one thing that I was really struggling with was making the galaxy feel active enough, that there was stuff going on without it being too threatening constantly to the human players, and astro trains were one way I tried to combat that.
2. They also cause disruptions to some wormholes so that it is harder to mine/turret those wormholes (thus providing more variety between wormholes, and a bit more complexity to wormhole defenses -- plus another thing to potentially raid into hostile enemy territory to kill).
That's mainly it. They provide some semi-predictable attrition along certain paths, and thus help to give a certain shaping and feel to the terrain. They're less vital now than they once were, and some players really hate them, but a number of reviewers have also commented on really liking them.
Is that what you were looking for, explanation-wise?