A gamer over at the official Eidos forums created a "trainer" file that opens up new areas in the TRA Demo! All you have to do is download THIS FILE
then unzip it. (As always do this at your own risk, I found no problems with it though) Follow these instructions which are in the Read Me file included with the "trainer" program:Instructions:
1. Execute trademoexp2.exe and then TR:A demo.
2. When you get to the checkpoint just before raptors, press Ctrl+1, then Ctrl+2 and Ctrl+3.
3. Get the cog and go back to the machine. If you fall down on your way back, go to point 2 ;)
You can try using Ctrl+1, Ctrl+2 and Ctrl+3 in other locations, but it will probably result in Lara dying.
You are using this trainer at your own risk!
If you're a little confused like I was about all this, go here
and scroll down a little bit to find a tutorial on how to do this. Trust me it's worth it! This trick allows you to play the entire Lost Valley level, including getting the third and last)cog. And is it ever awesome! I can't tell you how cool it is to play the level I remember from the original game, only 10x better! You'll explore the buildings surrounding the Lost Valley, and find all kinds of cool things (like huge Dinosaur nests!).
You'll even be able to access the gated area under the waterfall current that you opened with that switch at the beginning of the level. Inside you'll find this awesome relic!
So what are you waiting for, download the trainer file, and play some more TRA! If you have any questions leave a comment and I'll answer it.