But this wasn't the case — Space Age isn't only new content, but a complete re-balance of the original game. It is far far less grindy and requires much less baby-sitting of production.
Your first space platform, the ships, and the planets, are best thought of as unique Factorio-inspired puzzles. Each planet is like a Factorio game-mode to solve, with its own restrictions to design around.
I think those who have hundreds of hours in the base-game have to un-learn of the base game to pick up the DLC. Many of the complaints are pointed at the tech tree changes — they wanted an expansion on what was there, not a recreation. But for me now, I wouldn't recommend Factorio without Space Age.
All in all, the developers have a very different vision of what makes the game fun than I do, and that meant the expansion wasn't much fun for me. If I play the game more in the future, I'll probably do so with the expansion disabled.
I agree that no quality or endgame upgrades for trains is an odd choice, but I guess that's what the mods are for.
That said, I was more than happy to build the base back from scratch in space age, and I find the expansion to be every bit as fun as the base game. So I endorse it. Especially as you already know how to do some things quicker.