They were strange passageways, one on each side of the Blvd., that kind of twisted around and ran in an open fenced-off area next to the auto roadway at the bottom, as can be seen in this picture [[and glimpsed fleetingly in the movie posted by 48307 above):


[[Photos from Detroit Transit History site)

By the time I was going to middle school nearby, in the early '70s, that underpass had become a dank, damp, and dark place that we dared each other to go into. The stairways were crumbling, and there was clear evidence of people hanging out and perhaps sleeping down there. The city closed it off not long thereafter.

And, just for fun, a picture of the building of the vehicle underpass [[which we always called the "Belle Isle Tunnel", or, apparently erroneously, the "Jefferson viaduct") with one of Electric Park's rollercoasters in the background.

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