A "Train" is "one or more engines, coupled, displaying markers, with or without cars." Markers, on the rear, were essential to be an official "train." These could be either lights (day or night) or red flags (by day).
Santa Fe was another that used amber or yellow instead of green on their markers. And this practice on the Rio Grande and the Santa Fe changed back and forth over the years. The Rio Grande and Rio Grande Southern changed from green to yellow in the late 30's, I believe for the last time. Santa Fe had yellow up to the replacement of the twin marker lamps with a single centered red on the cabooses in the late 60's. From there it was all the way down hill to the "FRED."