A Primer on B&O CPL Signals - Good introduction to the basics of the CPL signal.

Absolute Permissive Block System - System widely used on former B&O lines.

Basics of North American Signaling - General site on U.S. signal systems.

General Signals - Lenses, lights and other items.

Railroad Signaling - General interest site with some CPL photos.

Historical Reference to Railroad Signaling - Great image section.

Railroad Signaling.com - Great image section.

RR Signal Pix- Scrap dealer and collector personal site. Nice CPL photos.