Past and present tries at pushing trains by controlling air pressure
One of the biggest issues with designing both large and small vehicles is something we can’t normally see— the air. As cars, trains and planes speed up, the amount of energy that has to go into pushing their way through all the air molecules in their path adds up, greatly reducing the vehicle’s efficiency. While most vehicles today adopt aerodynamic shapes to minimize this problem, there was a movement in the 1800s to try to utilize air pressure, turning it into a form of propulsion for subways and trains in particular.
Mr. Beach tunnels under Broadway
The most famous attempt at an air-driven train in the United States was called the Beach Pneumatic Transit, open from 1870 to 1873. Smaller pneumatic tubes were already in use to move small parcels over shorter distances, so the idea that a strong flow of air in a single direction could push an object wasn’t unheard of at the time. The questions were if something as large as a train car could be practically built, and if Alfred Ely Beach could get permission to even try building it.
Beach’s first major obstacles weren’t physics, but politics. He couldn’t get permits to build even a small scale pneumatic train from the city of New York, but felt that if people could see the railway in action they would support the expansion of the system. To start building, he applied for permits to build two small pneumatic tubes to deliver mail, later amending those permits to build one tunnel supposedly housing the two pneumatic tubes. This sneaky construction project continued for two years, with workers digging the tunnel behind a large metal shield, laying brickwork behind it as it progressed (not unlike modern tunnel boring machines.) To check their positioning, workers would extend a rod up through the dirt at night, with a person waiting above to see where it poked up in the street.
On February 26, 1870, the Beach Pneumatic Railway opened to a decently warm reception. The single car could carry up to 18 passengers all of 300 feet below Broadway, but was more of a proof-of-concept more than a form of functional transport. To make the limited demonstration more attractive, Beach made sure the car and station were attractively decorated, even including a goldfish fountain, which succeeded in winning positive write-ups from the press. In it’s three years of operation, the railway carried close to a half a million riders at twenty-five cents a ride, but it wasn’t enough to lead to an expansion of the project. Even if giant fans could get a railway car moving, Beach never gained enough momentum with politicians or investors. The tunnel was closed and eventually demolished, later being completely eclipsed by electric subway trains.
Pushing pneumatics past novelty
While Beach’s pneumatic railway never grew into a real mass transit system, it was not the only attempt at the concept of an air-propelled vehicle. Many functional demonstrations of pneumatic railways have been built, and while most also failed to be realized as a real travel network, they did allow for some engineering innovations on the concept. The Beach railway pressurized the entire tunnel, with air pushing the car from behind into a space with less air pressure (and thus, resistance), but engineers eventually realized that air pressure could be utilized even in less controlled environments.
Instead of pressurizing the entire tunnel, atmospheric railways run over a single pressurized tube underneath the train. The top of the tube has a slot with a resealable flap so that a plate can extend from the bottom of the train into the flow of air in the tube. That plate then acts like an upside-down sail on a ship, being pushed by the pressurized air being blown through the tube. This form of propulsion offers the some of the benefits of completely pneumatic railways, such only requiring a centralized, more efficient engine to pressurize the air in the tube, making the trains lighter and quieter without motors of their own. Aeromovel trains, found in Indonesia and Brazil, successfully operate in this way today.
The trade-off of the smaller tubes is that atmospheric railways still run into air that can slow them down. To really remove air resistance, you either need to be operating where there is no atmosphere to speak of, or by somehow getting that air out of the way. Beach achieved this idea to an extent back in 1870, but it’s now being developed again by Hyperloop One. Aside from the various material and technological advantages made available in the last hundred years, Hyperloop One isn’t looking to have their ‘pods’ be propelled by air pressure. Instead, they’re borrowing from maglev train design, moving the vehicle by changing the magnetic resistance along the track. The tracks are sealed in tubes primarily to have a space without air pressure, allowing the pods to reach speeds of up to 600 miles-per-hour. This system isn’t open yet, but hopefully they remember the goldfish fountain for opening day.
Source: Beach Pneumatic Transit, NYC Subway.org