Not all dungeons have walls.
A dungeon is really an encounter graph with connections between them. Encounter graphs can be generated without any walls.
You can start an encounter graph with a goal or a hook. A goal is an "end room" to your encounter graph (the treasure room of the moon king) while a hook is an entrance room to your encounter graph (a strange door with symbols on it on the side of a hill, glowing with magic as the moon shines on it).
In a dungeon, the graph follows the shape of the tunnels in the ground. Without that, you have to find other means to connect encounters.
One way is to connect them with information. You can see this in many films; the protagonists want to do X. One or more of them comes up with an idea (knowledge check). Some are ruled out, and some are attempted. Which leads to an encounter.
That encounter leads to other nodes. Often there are explicit choices.
Railroading is avoiding by making sure there is always more than one choice available, and more than one route to the goal (which could be a mirage) or to resolve the plot. Some of the choices could result in dead ends/side quests/plot hooks for another adventure.
We might start with a hook that implies a goal; a contract from an old contact to smuggle some cargo to old Earth.
[Contract (hook)]
...
[Deliver cargo]
now, old Earth has some security. So we have an intermediary goal:
[Contract (hook)]
...
[Find a way to avoid customs]
...
[Deliver cargo]
Maybe there are a few ways; corruption (bribe your way past customs), hacking (convince Earth security that you are cleared) and sneaking (get through customs without anyone noticing you).
[Contract (hook)]
...
Means of: [Corruption] [Hacking] [Sneaking]
...
[Deliver cargo]
In each of those cases, you are going to have to get information. Either find the right way to provide bribes, the right hacker or ice to penetrate Earth security, or a hole in the Earth custom's security fence.
[Contract (hook)]
|
+----------------+---------------+
| | |
[Government contact] [1337 Haxx0r] [Smuggler's Asteroid]
...
[Corruption] [Hacking] [Sneaking]
...
[Deliver cargo]
We can now drill down to one of them. The government contact doesn't want to communicate electronically, so arranges to meet you at a resort.
[Contract]
|
[Resort]
|
[Government contact]
?
[Corruption]
at the resort, shenanigans can ensue.
The Haxx0r needs access to a specific security module.
[Contract]
|
[Haxx0r]
|
[Security module]
?
[Hack Earth Security]
those security modules are found on Earth cruisers. But you don't have the firepower to take one out; however, a cruiser was lost in the Styggian Abyss and not yet recovered.
[Contract]
|
[Haxx0r]
|
[Cruiser in Stygian Abyss]
|
[Security module]
?
[Hack Earth Security]
but where is it? Well, you have a government contact; or, you could find an underworld contact who might know where it is.
The government contract might only work if you can get past the formal customs barrier, which requires sneaking through it. So you'll also have to do the Smuggler's asteroid subquest.
The smuggler's asteroid can have its own encounter graph.
The goal now is a series of options. Each of them actually ends up requiring you to explore 2/3 of the encounter graph (sometimes we recycle parts; if you go corruption, the government official is for finding someone to bribe; if you go haxx0r, the same official encounter graph (if not the same official) is for finding the location of the cruiser).
Keep adding choices and complications to the graph until the "dungeon" is large enough. Include mini-graphs (for the resort, the smuggler's asteroid, and for the cruiser); these mini-graphs can be reused for a related purpose later (much like you might recycle a dungeon layout you didn't use).
There are tracks, but there isn't a railroad, because there is more than one choice. You inform the players of their predicted results of going one step down each path, but not always about later consequences. And maybe some of the choices are dead ends (the smuggling route won't work, because the smuggler boss is maintaining strict secrecy about the hidden routes into Earth space, but by going that way you do a favour for a crime boss they aren't able to repay; treasure)