Rio Grande Signals
Mark Hemphill wrote on August 9, 2018:
First -- ALL signals that use colors and lights to provide an indication are color-lights, whether they use a fixed colored lens for each signal head, or a movable lens, aka a "searchlight"
D&RGW commenced its block signaling with GRS Type D color-lights, typically 3 heads, green on top, yellow middle, red beneath. D&RGW used Absolute Permissive Block (APB-ABS), the current modern standard in the late 1920s.
About 1934, D&RGW switched to GRS Type SA "searchlight" style color-lights, single-head. The first use of this signal type for wayside signals was the double track between Florence and Canon City, followed by the Joint Line, and then the Moffat. When D&RGW began installing CTC, it used SA dwarf searchlights at ends of sidings -- in APB-ABS there is no end-of-siding signal as the switch is not powered and the signal advises of track conditions ahead; it doesn't convey authority. Many subsequent signal installations that were inserted into existing Type D territory used Type SA because they were in inventory and Type D were not. Also, to further complicate things, D&RGW interlockings typically used Type SA.
So to answer the question of what was most common, Type D or Type SA, depends where you are. If you are looking at an original Type D territory, which is most of the main line from Pueblo to Salt Lake City, it's far and away Type D. If you are looking at an original Type SA territory, which is the Moffat, the Joint Line, and most of the interlockings, it's far and away Type SA.
There were some dwarf two-head signals on D&RGW, but not many.