How do I get my proposed terrain to show up in the same view window as my cross section and my vertical alignment?

It should be as simple as setting your desired terrain to be the Active terrain. Have you tried that?

I haven't.

As long as you have a terrain from the other corridor, I think you can reference that in and set that terrain as Active. Active terrain should show up in your dynamic section and profile views.

I'm less certain if the other corridor would show up if you referenced the Default-3D of that into the Default-3D of your active file. It might, but that might not be what you need.

You may also have the option to reference the other corridor into your file and generate Profile Intersection points from the other feature lines to tie your profile revisions into.

I'm not using ORD in production yet, so I haven't run into any really complicated situations yet.

So, the problem I'm running into is that I think I have the right file referenced in, but I can't see where the terrain shows up in the main view. I've tried turning off all the other levels, but I still don't see it. The thing is, I know there's something in that file because someone helped me with this before. They got it to show up in my vertical alignment, but I don't remember how they did it. I had to start from scratch because I didn't start my project from the seed file (I also put too much on one file) and it was lagging/ having issues.

Is that something that would only show up in my 3D view? Is that why I can't see it?

So, the problem I'm running into is that I think I have the right file referenced in, but I can't see where the terrain shows up in the main view. I've tried turning off all the other levels, but I still don't see it. The thing is, I know there's something in that file because someone helped me with this before. They got it to show up in my vertical alignment, but I don't remember how they did it. I had to start from scratch because I didn't start my project from the seed file (I also put too much on one file) and it was lagging/ having issues.

Is that something that would only show up in my 3D view? Is that why I can't see it?

Yes, the terrain should be attached to the 3D view. ORD can generally manage it from there.
Since a terrain is a 3D element in a 3D file, it needs to be referenced to the Default-3D model.
At least, that's my understanding.

It doesn't look like there's anything in my 3D file, which is weird because the references are referenced in and turned on. It's not usually like this. This is seems like a weird problem.

And I have no idea why, but every time I go from my 3D model to my Default model, all of the view windows except view 1 and view 2 (which shows up blank) vanish, and I have to set up the views again.