I stumbled across "The Position Light" bog earlier this morning and was amazed at all of the information it contains regarding all aspects of railroad signalling. I was searching for locking devices on CTC panels and a picture of Amtrak's THORN Tower appeared in the images page. Here's a link to the blog post: http://position-light.blogspot.com/2011/12/amtrak-thorn-interlocking.html. There's an incredible wealth of information in this post on the operations of THORN Tower along with a large number of photos and diagrams. And the home page for the blog is now listed in the Great Railroad Modeling Sites panel on the right of this blog, too. Absolutely great stuff!
For those of you who have interest at all in railroad signals, spend a little time on this blog.