This place at 125th Street was about my only chance to stand right over the conduit slot and take my time getting a photograph looking down into it, without feeling like some speeding car would be on me in seconds. Anyway, it looks like it's pretty well filled up with garbage by now.
We'll finish with the mysterious hatch, which can be seen in the lower photo. It looks like there may have been a larger opening opposite it, extending the whole way from the conduit to the running rail (and beyond?), since at exactly that point there are no paving stones but rather a blacktop patch. So what is it?
UPDATE... May 27... I'd been wondering whether this might have been a plough pit, a place where someone could go down under the cars and put ploughs on and off, but maybe not. This one looks like it allows good access into the conduit, but I think the cleaning manholes are the large covers we saw before..