I’m bewildered by some signals I see on the UP. So on a signal head you usually see three lights as follows:
o - green
o - yellow
o - red
But on the UP I see signal heads with four lights and I have no clue what the other aspect is. I’m assuming lunar but never seen it lit. It would be in the pattern of:
o - green
o - yellow
o - (what is this one?)
o - red
Educate me 
Well nobody has commented and I know nothing about the UP so I can only speculate since the PRR had a similar situation. The PRR had a light mounted lower on the signal mast that could be lit to allow a train climbing a grade to pass a red signal IF it would stall on the grade otherwise. There were serious speed restrictions associated with the maneuver and the engineer had to be able to avoid a collision knowing there was another train in the block ahead. It could be to allow something similar.
I know just what you are talking about and I am unsure also. I do have an idea though, it may be white (also called lunar to some railfans). On former B&O signals it indicated speed restrictions. On modern CSX signals I’ve seen them being used to indicate a train meet.
If I am wrong on this I would like to be corrected, but I am pretty sure!
it isnt just used by railfans…that color is called lunar white… when used in a traffic like style signal head it shows the indication of restricting… aka prossed at restricted prossed…
csx engineer
I run on the UP and have yet to see one. Show me a photo…
http://www.alkrug.vcn.com/rrfacts/signals/signals.htm
this might give you alittle info…im trying to find a traffic light style singal pic with the lunar white light…
csx engineer
Not aspect, but light. The aspect is the totality of what’s lit, such as red-over-green, flashing yellow, etc. The fourth head is usually another yellow light, so the signal can provide a yellow-over-yellow aspect, which is a diverging route indication.
There are hundreds if not thousands of lunar lights on UP. The most frequent use is the entering signal for a non-bonded siding, where the best possible aspect is red-over-lunar, indicating diverging route, restricted speed. Many of the sidings on the former Western Pacific and Oregon Short Line are non-bonded.
RWM