A reference page that provides extensive background information about railroad signaling. There are many good images with explanatory material that cover signal systems, semaphore signals and then methods for adding signals to a model railroad layout.
Tutorial Train Signals – Excellent well illustrated basic presentation.
Railway Aspects And Signal Indicators – Long white paper that is a good basic reference.
Steam Community Images – This is an excellent visual presentation of signalling from the basic to the advanced.
NORAC Signaling system – In depth discussion with video and images.
US Railroad Signalling – Mostly a text discussion.
How To Use Signals On Model Railways – Discusses making semaphore signals
Semaphore signalling explained – A google search page with many signalling images.
Adding Signals To A Model Railroad Layout
DIY Arduino Model Railroad Multiple Block Signal System – DIY Digial Road beginning of a good series.
Basic Signaling Part 1 – Good info on commercial systems. Digs deep into the atlas system.