7. 42nd Street A/C/E at Port Authority
It’s unclear why the the lower level of the A/C/E track was constructed, as it never operated as part of the IND or IRT systems but it was used between 1959 and 1981 as the origin for the special Aqueduct Racetrack train.
According to NYCSubway, the platform was on the southbound side, “one track, underneath the downtown local track on the upper level, and one side platform underneath the island platform.” The entrances were all sealed, except for one under a trap door on the “south end of the southbound platform,” at the time Peter Doughtery was writing. Today the tunnels beneath Port Authority have been converted into the 7 line subway extension, of which you can see the progress here in our behind-the-scenes photos of the construction.