My brother in law's parents run an antiques store and someone brought in some old Trains magazines. Included in them was a small booklet/mini-magazine called "All About Signals" from 1957 that is an excellent write-up with lots of diagrams on the history and usage of signal systems over the years.
I scanned it to PDF and posted it on the CSPPRY website. Feel free to download it here:
http://www.csppry.org/AllAboutSignals.pdf (21mb)