Okay, sadly, I still live and breath Outlaws, so I know what you're talking about--and it's not in any walkthrough that I know of. It's not important to the level at all; it's just one giant easter egg. IIRC, the only
way you can get into that room is in a short span of time right after you throw the switches to open the big door. I made it inside by sprinting over, so I have no idea how long the door stays open for.
If you've already thrown the switches, you'll have to revert to a saved game or start the level over to see what's inside. Or you could just check out the write-up I did when I first found it
. (Shameless plug? What?) Although it does kind of have to be seen to be believed...