There is a
tunnel portal for the old
subway visible from I-75. It is in a retaining wall in a steep
hillside on the East side of the
freeway just south of all the exits for all the viaducts and right after Central Parkway stops going along I-75. At this point the subway would have emerged from underground, and would have run at grade along the Mill Creek valley for a bit, which is now home to I-75.
The original plan was for a single circlular subway line, using the covered canal bed discribed above and emerging at the portals along I-75, continuing north at grade level and then veering east, approximately following the route of the present day Norwood Lateral. Then it would have ran at grade through Oakley, Hyde Park, and East Walnut Hills, before descending downtown on an elevated concrete trestle. Then it would have run underground through downtown back to the canal bed section. Only the canal section was completed. Interestingly, the city of Newark did build a functioning subway using an abandoned canal bed.
A postscript to the story of the Cincinnati subway is as follows:
The streetcar lines of Cincinnati were torn up beginning in the 30s, due to some choice bribes from General Motors, including the spectacular unique lifts that took said streetcars up the hillsides. After the Second World War, much of the city was torn down to make way for freeways and suburbanization commenced at a rapid rate. While the metro bus system operated at a level somewhat better than bus systems in other cities, public transportation was and is basically a joke.
There has been perpetual talk in the last 15 years of building a light rail line or two, and this now seems somewhat likely.