I have no substantive backup for my understanding. So with that said, my understanding was/is that the fastest tracks were ballasted higher than the less used tracks. Hence Dispatchers would use the term to inform various highspeed trains to "Take the Highrail". I think this is indicative of the New York Central "Water Level" route where there were four tracks with the Middle two at a higher level. Pick apart my explanation to your heart's content.
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