Elon Musk's Hyperloop announcement resulted in quite a bit of skepticism. We'd like to think that has less to do with the feasibility of Musk's concept and more to do with the massive mass transit failures of the past. And there have been some doozies. A few outlandish projects were so science fiction that they were doomed from the start, while other proposals were possible, but died an ignoble death due to financing or technical difficulties. Regardless, these public transit pipe dreams are cautionary tales for those who dream of a world that's easier to navigate. SkyTran SkyTran, a system of two-person pods hanging from a suspended maglev track, falls under the realm of Personal Rapid Transit (PRT). Most PRT systems are expensive, utopian dreams, as they're only slightly more efficient at moving people than an automobile. Plus, they cost far more per mile to both build and operate than most other forms of public transit. SkyTran was supposed to be different. The brainchild of Douglas Malewicki, SkyTran promised automation and energy efficiency. But despite numerous proposals over the course of three decades, only a crude prototype has been built. There's a chance SkyTran may eventually hover over the crowded streets of Tel Aviv, but it'll remain on this list until it's operational. Photo: Skytran

Boeing 2707 Convinced that supersonic travel was the way of the future, and worried that European development of the Concorde would leave the U.S. aircraft industry in the dust, President Kennedy introduced the National Supersonic Transport initiative in June of 1963. Boeing quickly got to work on the 2707, an advanced supersonic aircraft with seating for 277 and much larger than the Concorde. But funding cuts and environmental worries led to the project's cancellation in 1971. Two prototypes were developed, but never finished. Today, no supersonic aircraft carry passengers. The 2707's memory lives on in the name of Seattle's erstwhile basketball team and airfoil developments that were later carried over to production aircraft. Also, R.I.P., Concorde. Photo: Boeing

Beach Pneumatic Subway No, this has nothing to do with shuttling suntanned tourists across the sand. In 1870, a 300-foot subway ran beneath Broadway in New York City. It was the creation of inventor Alfred Ely Beach, and it was originally intended for package delivery. But Beech had other plans, secretly building stations and subway cars capable of carrying people. Cars were propelled by a vacuum blower that pushed the car through the tunnel -- similar to the system used at bank drive-up windows. The demonstration subway ran for three years, with political opposition and an economic downturn dooming it to failure. Building owners on Broadway didn't want a subway tunnel to potentially damage foundations, and elevated transit was cheaper to build. The station burned in 1898, and the MTA's City Hall station now stands where Beach's experiment died. Photo: Joseph Brennan

Passenger Airships Before World War I, airships regularly completed passenger flights within Europe for DELAG, the world's first revenue passenger airline. Though airships were used during the war for reconnaissance in the North Sea and devastating bombing raids on London, they were back in passenger service by the early 1930s. Transatlantic crossings became quite popular despite the Great Depression, and then there was that whole Hindenburg thing. Between that and the onset of the Second World War, the airship era ended. Photo: Getty Images / Archive Holdings Inc.

Schweeb Around 2009, a pedal-powered monorail system with the ridiculously creepy name Schweeb started making the rounds in the design and tech circuits. It gained support from Google, but that didn't help the fact that combining a monorail and a recumbent bicycle is patently ridiculous. It's got all the infrastructure challenges of PRT, but it's only usable by those without physical limitations. It's no wonder a Schweeb system isn't currently carrying passengers. Photo: Schweeb

Hotchkiss Bicycle Railroad Similar to the Schweeb, the Hotchkiss Bicycle Railroad put a bicycle on a monorail. Examples were actually operational between 1892 and 1909 in both the U.K. and U.S. The most famous example, the Mount Holly and Smithville Bicycle Railway, sounds like the name of a 70's-era bubblegum band, but it was built to carry employees between the town of Mount Holly, NJ and a factory in Smithville. Unfortunately, a single track meant that passing was impossible. When two cyclists met coming from opposite directions, one of them had to pedal back to the beginning of the line. Whoops. Photo: Burlington County Historical Society

Straddling Bus In 2010, the U.S. Elevated High-Speed Bus Group promised that giant, traffic-straddling, solar-powered buses would soon glide over traffic jams. Thankfully, nothing has come of that promise. A designer must've realized that a bus tall enough to glide above trucks would take out bridges and streetlights, and a giant bus speeding over traffic could turn a simple right turn into a disaster of Michael Bay proportions. Photo: US Elevated High-Speed Bus Group

Railplane Before Benny and the Jets, there was Bennie and his Railplane. Inventor George Bennie envisioned propeller-powered locomotives moving passenger trains on tracks suspended above slow-moving freight trains. He got a prototype running for about 400 feet in the mid-1930s, but apparently investors thought the idea was too far-fetched, and Bennie ended up bankrupt right before the onset of World War II. Photo: Wikicommons

Scramjet If you're stuck in the middle seat between a loud snorer and a screaming toddler, traveling 5.1 times the speed of sound may seem like an appealing option. In theory, that's possible from a scramjet -- an aircraft that uses supersonic airflow through the engine to maintain extremely high speeds. In fact, military tests of scramjets have been successful as recently as this summer. Unfortunately, scramjets can only operate at supersonic speeds—they can't accelerate up to them. That means a scramjet must be carried into the air by another aircraft and released. Drag, heat, and maneuverability are also all major concerns while blasting through the air at Mach 12, which is why a passenger scramjet is highly unlikely in the near future. If scramjets ever do make it to production, they'd likely be unmanned aircraft used to intercept enemy missiles. In other words, a business class upgrade is highly unlikely. Photo: Nasa Rendering