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Every 105 feet-- because it's a multiple of the 15 feet between insulator manholes-- there was a cleaning manhole larger than the insulator manholes, although still less than 3 feet across, and it is not located between the rails but along the outer side of one running rail.
UPDATE...May 27... I'm told there were scraper cars that pushed along street litter in the conduit up to the cleaning manholes, where it could be shovelled out. I now think that's what these two are, but I'm not so sure. Every fourth one (420 feet) was a little larger, and called a feeder manhole because it also provided access to the electrical feeder running underneath the contact rails.
These are harder to find exposed. The one at 113th Street actually has a hole in it, and the sunlight did not illuminate the bottom of the space underneath. It says "3 AVE" somewhat like the conduit manhole covers.
The only other one of these I could find is up near 122d Street and its cover has a different design.