(Portland, OR) -- The Centennial Mills building in Northwest Portland is a concrete skeleton with a huge metal water tank on top after last week's massive fire. It's in danger of collapse. The city doesn't want Union Pacific to run freight trains on the nearby tracks out of concern the vibrations could cause the building to come down. UP parked a buffer train filled with rocks to protect other trains, if there is a collapse. The city is setting up a camera to monitor changes in the building that might indicate it's closer to falling. Demolition could happen as soon as Friday.