Some photos of an abandoned subway in Cincinnati just showed up online...

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.

The incomplete Cincinnati line sat fallow through the Great Depression and WWII.  Bridges, stations, and retaining walls along the surface stretches deteriorated to such an extent that a few items actually collapsed.  Nearly everything above ground was bulldozed to make way for portions of I-75 and the Norwood Lateral in the 1950's and 1970's, respectively. The mute two mile tunnel that remains under Central Parkway is unknown to many Cincinnati natives, and what most who do know of it know consists largely of hearsay and speculation.  http://www.cincinnati-transit.net/subway.html

Seems like quite a waste of money and space... I was thinking... and I was right.

From Wikipedia:
a set of incomplete, derelict tunnels and stations for a rapid transit system beneath the streets of Cincinnati, Ohio. It is recognized as the largest abandoned subway tunnel in the United States.

 It was built in the early twentieth century as an upgrade to the Cincinnati streetcar system, but was abandoned due to escalating costs and collapse of funding amidst political bickering during the 1920s. http://en.wikipedia.org/wiki/Cincinnati_Subway

From 1825 until 1920 the Miami and Erie Canal divided Cincinnati's residential neighborhood of Over-the-Rhine from the business district of downtown. The canal was used to transport goods and people from the Great Lakes to the Ohio River and subsequently the Mississippi River via Cincinnati, until the popularity of railroads caused it to become disused. The canal became unprofitable by 1856 and was officially abandoned by the city in 1877. On September 27, 1884 a weekly Cincinnati magazine called The Graphic proposed that the "dead old ditch" be used to provide an unobstructed route for a subway system, with a large boulevard above.

In 1888, Cincinnati began adopting electric streetcars, which soon became the main form of public transportation. During this period Cincinnati was one of the 7 most-populous US cities and had a rate of growth and economic importance that was similar to that of New York City and Chicago. The slow streetcars shared the crowded streets with horse-drawn carriages, people, and collided with the first automobiles on an almost daily basis. It was not unusual for trips between downtown and the surrounding suburbs to take 45 minutes to an hour. Despite having 222 miles of streetcar tracks, the city found itself in a growing traffic nightmare.

The next year City Council convinced the Ohio State Legislature to lease the city's portion of the canal for use as a boulevard and subway system. The city hired experts that worked on Boston's and Chicago's Rapid Transit systems to research the best possible implementation for Cincinnati's Rapid Transit system.

By 1919 the cost of construction had doubled,any hope of raising the money to complete the subway was further delayed with the stock market crash of 1929

The boulevard that ran on top of the subway, called "Central Parkway," officially opened to traffic on October 1, 1928, and was followed by a week of public celebration.

With cheap inexpensive automobiles becoming commonplace, and the need for a subway and street car system no longer a priority, it was abandoned.

In the 1950s a massive 52-inch water main was laid in the north-bound tunnel to save $300,000 in construction costs.

In the 1980s the city pitched the tunnels to Hollywood filmmakers as an ideal location to shoot subway scenes. In particular, the location was presented to the makers of Batman Forever, but as of 2008 the tunnels have not been used in any feature films.

Recently, the uncompleted subway tunnels and stations have been described as "strikingly well preserved" and "in good shape." This is partially credited to the original construction quality, and partially because Cincinnati must use tax dollars to maintain the tunnel because Central Parkway is situated on top of it.

As of 2008 it would cost $2.6 million over five years to maintain the tunnels as they currently are, $19 million to fill the tunnels with dirt, and $100.5 million to revive the tunnels for modern subway use. Relocating the 52-inch water main would cost $14 million.

A similar construction can also be found at the Newark City Subway, which actively uses the former bed of the Morris Canal and the Rochester Industrial and Rapid Transit Railway, which once used the bed of the previously abandoned Erie Canal.