As you can see, the floor is completely black when it should have a gray carpet.

Originally this thread was asking how to fix this, but I figured it out on my own shortly after posting.
For those still wondering like I was, here's the way to fix it: Open the instance for your elevator base, select the static props on the floor inside (the ring around the base and the piece in the center), and disable shadows on them. I don't know if the center bit actually affects it, but the ring definitely does.
Disabling shadows on other props may be a good idea as well, to eliminate other issues. Experiment and find which ones you think improve the look the most.

Additionally, if you disable shadows on the square frames on the outside surrounding the staircase leading in/out, you can eliminate the stupid shadows there as well and end up with an elevator looking a lot more like one of Valve's.

Honestly, why Valve wouldn't set this all up correctly to begin with is beyond me...

Only 2 things:
-Or is a instance problem.
-Or is a texture problem (I think).

The lighting is ok because the rest of the models are ok, so I think it can load correctly the floor's model.

Vordwann wrote:
Is it the old aperture one? Because if so, just remake the trigger at the bottom of the elevator and then make it start the tracktrain forward...

No, I read your tip about the old aperture one on another thread. It's one of the partially destroyed ones in the modern aperture setting.
I've seen the problem in a few other maps I've played. Usually reloading the map a few times helps. And I have also confirmed that it's not just me. I had one of my friends test out the map and he told me about the problem too. If it's the first map I load since starting up the game, it almost always works. But if I've had Portal 2 on for a little while and loaded a lot of maps, it's a lot less likely to work, even after multiple loads of the map.
Twin Pillars is the map if you're that intent on finding out what the problem is. The .vmf is included in the download.

NunamedDragon wrote:
I know waht the problem is You need to compile the map with the final (slow) compile setting

I just tried that last night. It seemed to help the problem in that the elevator is more likely to start, but it didn't solve it. Also, the final compile screwed something else up. When I play the map, the portalable surfaces I have behind metal grating just show up as a blank black texture! It doesn't happen when I compile the map again with the default compile settings. So some help on the elevator would still be appreciated. But what about the grating? Do I have to move it farther away from the wall? I currently have two units in between the grate and the wall.

Welsh Mullet wrote:
I think the problem is that using the instance elevators, the entrance one's projected texture isn't killed, so the exit's one doesn't work

When you setup your first door after exiting the entry elevator select the trigger that makes the door open and have it kill the env_projected texture in the elevator instance just before opening the door... Otherwise all the other env_projectedtexture's won't work (including the one in the exit elevator)