The abandoned area is a section that connects the steelyard and The Mill in The Pitt.
It was abandoned by the Pitt raiders and slaves when trogs started moving into the area, but it's still used to get to the steelyard, so the slaves were made to construct a barrier so that other slaves could get to the steelyard safely.
The abandoned area is a medium sized room with some caged walkways in it in order to get to the steelyard in one piece. There is a ripped open gate that let trogs ino the area. A couple of trogs may spawn outside the fencing but sometimes you can sneak past them.
The abandoned area appears in the Fallout 3 add-on The Pitt.