The system was never completed but the tunnels still exist. Go to this pretty cool site to see more.
Abandoned tunnels are often the object of urban legend, but Cincinnati is in fact the site of the country's largest abandoned subway tunnel. But "abandoned" is not quite the word, as construction slowed to a stop in 1925 before even half of the 16 mile line was completed. Seven miles between Cincinnati's central business district and the industrial suburb of Norwood were tunneled, bridged, or graded, but no track was laid and no subway cars were ordered. No passengers ever rode between the six stations that were built.
Central Parkway construction, north of the Plum St. turn.
The finished Parkway, with subway beneath. The Race St. station is directly below this point.