Architect Craig Hodgetts' idea was the cover story of New York magazine back in February of 1969. He called his proposal the "Landliner." Hodgetts believed that the Landliner would be effective at servicing a long, linear city like New York:
"To service this linear city, the Landliner, inspired by the Staten Island Ferry, according to Hodgetts ("it was wide, short and stocky, rather than long and thin [like a] traditional train"). The Landliner would zip above existing roadways, rising on nearly friction-free air cushion bearings, and hovering above cars."
Hodgetts' Landliner would travel at light-rail speeds and have services on-board like gyms and bars.
Pictures over at Curbed LA.