Railroad Signals Landing Page
Under construction! (not really)
Some helpful and informative links:
Jovet’s Signal Rules Sheets
Al Krug Railroad Signals Explanation (Archive)
Carsten S. Lundsten’s Guides on North American Signaling Practices
Eric Haas’s Photography Site featuring several signal collections
The Signal Page – World-wide Signaling Resources
How U.S. railroad crossing signals should flash, viewed from above:
Virtual Railfan Waycross, GA Southbound Jesup Subdivision Jct Signal: