> Thus the train was propelled by air pressure [15psi]. Neat. It 
 > worked.
There was an experimental subway train in New York City that was propelled by 
vacuum also. The station, a car, and other remnants still exist, buried deep 
beneath the city streets. It failed due to political problems more than due 
to technical considerations. (well, at the time: it probably wouldn't have 
been a long-term success anyway.....)
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