For a cool preview of the Transit Museum Tour of the City Hall Station you can view this video I found on Youtube (there are two related videos by the same videographer)

http://www.youtube.com/watch?v=70xMudgyV-A

One piece of advice about this tour. It lasts about 45 minutes and my kids starting getting bored after 25 minutes. Bring a snack for them and possibly a lightweight stroller to sit in. THe station is extremely dirty and even touching the walls leads to very sooty hands etc and clothing. Be prepared to wash everything.

Also make sure to prepare the children that they MUST hold your hands. As seem by the video there are LARGE gaps at this station when trains come through. It was designed for smaller train lengths.