Oh no! It's worldbuilding!

While carrying on with Tom's story (1600s revenants & witchfinders), which is over halfway through, I've started muddling about with the Chimps on a Blimp story as the setting for a probably-YA novel. It's kind of relaxing to be able to move between stories, after the more-than-a-little intensive spells of revision of Willow Knot.
Tom's story is dead easy for world building, because it's Stuart England, barely tweaked. Sure, it has working magic, but since most people in 1600s England (and elsewhere) believed in magic, real magic doesn't change the society as much as one might think. I mean, there are legal and philosophical discussions about magic already in place.

With the short story (On the Transmontane Run With the Aerial Mail Express, plug, plug) I was able to do a fair bit of handwaving and 'quick, look over there!' to avoid setting-holes (like plot holes, but you can fall further and it's all mucky at the bottom) or at least direct attention away from unexamined areas. With a novel I can't do that nearly so much, just because the canvas is wider, and I'm not painting miniatures.
Also, there are things I will need to know about the world for my self, whether or not those aspects come directly into the story. What I'll need to know may not be what another writer in the same situation would need to know, and some of it I'll only realise as I write.

Obligatory link to Patricia Wrede's worldbuilding questions, both helpful and daunting.

The first and biggest question is: why no railroads? What makes airships commercially viable? In the short story the blimp travels over swamp and mountain, both difficult terrain for railways. But what about the plains? Do the rails run over the prairies, then stop as the foothills rise?
Then there's the weird ecology, the freshwater jellyfish that are a crucial plot point. And the flying squid, that don't show up in the short story, but might be real later on. I began to think longingly of Australia, and wonder about heaving some sort of Atlantis out of the sea, and supplying it with weird critters. Maybe I could crack North America down longways, put some massive waterway between prairies and Northwest coast.
I brought the question to the Scribblers, and bookherder said 'go and look at late Cretaceous maps'. Lo, there was the Western Interior Seaway, almost exactly as I'd pictured it.
Now I'm all excited, and need to find something like A Child's Illustrated Guide to Paleogeography, with fold-out maps. Research!
What happens if the prairies are covered with water? No giant buffalo herds, for one thing. How much of Europe should I allow? Does the land bridge last longer?
Such fun to come.