CTA workers recently repaired a section of track in the Red Line subway tunnel. A couple of years ago the half-ties in that area incurred minor heat damage from a track fire. Although damage to the ties was minor, and it was still safe to operate trains in the area, CTA implemented a slow zone until repairs could be made.
The CTA last week released a Flickr photo set of the work. I think the photos are stunning, and have put some of them in a photo gallery. But be sure to check out the full Flickr photo set.