While in the tunnels under the buildings, go to the long tunnel from the rail yard to the center of the tunnels. In the long tunnel, get to where a fence covers the pipes. Stand and back up to the fence, then jump. Just before landing, press B. Lay down and turn sideways, then slide into the fenced pipe area. You can crouch and go back and forth in the fenced off pipes. If you are using a small weapon (Uzi, P90, or MP5), no one can see you until you fire at them. Be careful with grenades; they must be thrown between the pipes.