If you are looking for more information on the types of signals used by MAX light rail trains in Portland & surrounding areas, this is your starting point.
Discussion about signals has spanned several posts because MAX uses several types of signaling systems, so here’s a consolidated index of links to help you find what you are looking for.
ABS Signals (Automatic Block Signal System)
ABS Signals Part 1: An introduction to blocks, signals, aspects, and indications.
ABS Signals Part 2: Builds on the information from Part 1, explains route indications
ABS Signals Part 3: How the signals are numbered, map of ABS territories
Pre-Empt Signals
Pre-empt Signals: Overview, explanation of pre-empt aspects and indications, map of pre-empt territories
Standard Operating Procedures to proceed when a permissive aspect is not displayed
ABS/Pre-Empt Combination Signals
ABS/Pre-empt Combination Signals: Overview, explanation of route indications, locations of where some of these signals are used.
Other
Dwarf Signals and C-Signals: Dwarf signals protect mainline power switches when trains are running reverse traffic in ABS territory. The C-signal sort of functions as a relay to let operators know their call went through at Skidmore Fountain westbound, since the pre-empt is not visible from the platform.
Camelopardalis
MAX FAQs 
So I was coming home today and spotted something I didn’t know what it’s purpose was. Was west bound, just before Beaverton TC on the corner of the east bound track was a single signal with a red only aspect on it which was lit. Is this signal for when trains have to run backwards on the track? Or is it because of the blind corner or….?
That’s a dwarf signal – it’s there to protect the power switches when trains are running reverse traffic
Ahh thanks. I’m sure I read that post but probably glazed over it as I had read a lot about signals by then and my brain was probably mash. :)