Now that you mention it, that escalator was very close to the southernmost staircase to the platform. I became intimately familiar with the 42nd St. mezzanine during the three years I was a Saturday commuter, and used that very staircase all the time. There used to be "Entrance to Aqueduct Specials at 40th Street" signs all along the mezzanine, and another sign marked, "Entrance to Aqueduct Subway Special" with an arrow pointing to the escalator. It was fenced off on Saturday mornings, and I alwayd had the impression it was abandoned.
I also remember when the northern end of the lower level was used as an underpass between both active platforms. In those days, the middle of the mezzanine was outside the paid fare zone. We were headed uptown once and accidentally went through the turnstiles to the downtown platform. I remembered the underpass and led our group to the northern end of the platform as an E train of R-38s departed on the express track. We went down the stairs and then I saw the track and platform through the chain link fence they had rigged up. It felt like a twilight zone and gave me the creeps.