@Someone_Else: welcome to the forums
Glad you've been enjoying the game. Those wiki articles are indeed a good place to go in figuring out what expansion has, but a few of the high points:
The Zenith Remnant: Golems. Lots of "more" in bonus ship types, starships, minor factions, special AI structures, etc.
Children of Neinzul: Hybrids. Younglings. A bit of more in other areas but it was a smaller expansion in general (and a smaller price tag).
Light of the Spire: The Fallen Spire campaign. Spirecraft. Lots of more in other areas.
Ancient Shadows: Champions. Dark Spire minor faction. Lots of more in other areas.
And all of them add more of the awesome music by Pablo Vega, which is one of my favorite things about the expansions. Ancient Shadows in particular has some really good music.
To put it another way: I'm not aware of anyone ever complaining that any of the expansions didn't contain enough value to justify the price (and the first three expansions have come down a lot in price since the beginning, even without the current promotional sale)
Hm, the Wiki lists the special astro trains for Zenith Remnant, since the base ATs are gone now what exactly does that mean for enabling Astro Trains in the base game?
The Speed Booster and Regenerator trains only show up if you have TZR enabled since that's what added them, the rest are base-game.