The subway's origin traces to an older failure. The Miami and Erie Canal once moved goods and people from the Great Lakes to the Ohio River through Cincinnati, but railroads made it obsolete. The canal became unprofitable by 1856 and was abandoned by the city in 1877, degenerating into a polluted dumping ground. When downtown congestion became unbearable in the early 1900s, city leaders saw the derelict canal bed as the perfect corridor for rapid transit. Mayor Henry T. Hunt championed the plan, and in 1916 voters approved a $6 million bond issue to build a subway loop. But World War I delayed construction, and when work finally began in 1920, postwar inflation had doubled the projected costs. The money that was supposed to build a complete system could now only manage a fraction.

Workers dug and graded over two miles of tunnel before the money ran out completely in 1927. The plans had called for twelve stations, from Oakley in the east to Crawford near Spring Grove Cemetery, with a stop beneath Fountain Square at the city's heart. None were finished. Construction cracked the foundations of buildings along the route, sparking lawsuits. In January 1929, Mayor Murray Seasongood dissolved the Rapid Transit Commission and took personal control of the project, but the stock market crash that October buried any hope of new funding. Critics dubbed the unfinished tunnels Cincinnati's White Elephant. Seasongood and many residents still expected construction to resume once the economy recovered. It never did.

The tunnels have been studied, proposed for, and rejected more times than any infrastructure project in American history. In 1936, the Engineers' Club of Cincinnati concluded the tunnels had no use beyond their original purpose and recommended they be forgotten. In 1939, they were deemed unsuitable for automobile traffic. In 1940, experts recommended placing all streetcar lines underground, but the city had too many other expensive projects. World War II shelved everything again, though old bunk beds found in the tunnels suggest they may have been quietly used as air raid shelters. After the war, the auto industry lobbied to use the tunnel right-of-way for Interstate 75 and the Norwood Lateral. In the 1950s, a massive water main was laid inside the northbound tunnel to save $300,000. In the 1980s, the city pitched the tunnels to Hollywood as a filming location for subway scenes, approaching the makers of Batman Forever, without success.

What makes the Cincinnati Subway haunting is how ready it was. The double-tracked tunnels run north and south, each half a generous width and height, separated by a concrete wall with openings for cross-passage. The curves are banked to accommodate trains at speed, with outside stringers raised higher than inner ones. At Mohawk Corner, the wall is set back to accommodate a station that was never built. At Walnut Street, the tracks begin curving south toward downtown before stopping abruptly at a bricked-up wall. The tunnels are well ventilated and naturally lit as far as Liberty Street. Engineering reports have consistently described the tunnels as in good shape. A 2002 proposal to use them for a regional light rail system, estimated at $2.6 billion over thirty years, was voted down by more than two to one.

In late 2024, the City of Cincinnati issued a new Request for Information seeking ideas to repurpose the tunnel, the latest in a century of attempts to find a purpose for what the city built and abandoned. The wooden stringers still wait, bolted to floors poured over a hundred years ago, in tunnels engineered for trains that exist only in blueprints. The subway bonds were paid off decades ago, but the tunnels remain city property, perfectly preserved beneath streets where cars carry the traffic that subway advocates once hoped to move underground. The Cincinnati Subway stands as one of America's great urban what-ifs, a monument not to what was built, but to what was almost built.