About the time this thread came up, I discovered the Otto Perry collection at the Denver Public Library. I spent mucho hours looking back and forth at trains as they moved hither and yon, and as my attention was drawn to this point, I saw as many as 20 photos, many of different train movements in different areas using this same caboose-before-locomotive helper positioning. Being green as green for such rules and regs, I would have thought nothing of these pix had this thread not come up.
BTW - Sam, there is a pic of a train ascending Cerro summit that Perry actually caught a telegraph pole with enough clarity to make out an original install CD 126 "blobtop" still in service in 1940, or so. He usually took steps to keep such "clutter" out of his train shots!