@Loose-Caboose posted:Hence Dispatchers would use the term to inform various highspeed trains to "Take the Highrail".
From CSX's site: Railroad Dictionary
High Iron: Main line or high speed track
High Rail: The outer or elevated rail of a curved track. Could also be an inspection vehicle equipped with rail wheels.
I think Tom's find of the 1946 Model Builder Magazine ad is the best since it is the earliest actual written reference so far.