A while back in a thread about signals the topic of LED signals came up. You mentioned something to the effect that the UP didn't use them because the colors "weren't approved/certified by the FRA". After some digging around, I find that Safetran guarantees them to be in compliance with:

Chromaticity: Meets AAR Manual Part 7.1.10
Beam pattern and values: Exceeds AAR manual part 3.2.37

Is this what you were referring to? I find no such guarantees in their catalog for the plastic and glass color lenses (Corning glass lenses from the 1940s/50s each had a signed inspector's sticker on them). Metrolink has quietly changed all its signals to LED over the past year or so. I did see or hear something about how the UP doesn't like LEDs because "unlike incandescent lamps they don't melt snow", as if the incandescent ones do, but I don't take much stock in the story.