In the Commonwealth Saga it's trains! It's portals with hugely demanding power consumption. They mostly have to stay fixed to one place and open. So they run choo choos. Their world is commerce and economics. And trains are a lovely symbol of that.

In The Final Architecture it's jaunty. Unspace helps you go fast but you are always alone. Crewmates gone. When you come out they reappear. When you inside there is something coming to get you. Something that lives in unspace and doesn't like that we use it for travel. The terror of its hunting you drives everyone to suicide. So instead they sleep. Magic "you sleep now" pods for everyone.

Except. You can only sleep if you are on a known route. Some rare people can feel out new routes. And they have to say awake. Most shows just follow normal routes. But the special ships with these other folks can go all over the place! At the cost of route terror.

The books are about coming together in the face of adversity cosmic horror. And unspace is a foil to that. You are alone. But we do what we can anyway. Your alone now, but not forever. Unless the monster gets you.