This is actually a Mobile Lounge, created to shuttle passengers directly from terminal to plane without ever touching the tarmac, and was a feature of the original airport design as conceived by Eero Saarinen. This was before the advent of the jet bridge that made this system obsolete, and decades before the construction of the Aerotrain and additional terminals at this particular airport. It's still used today as a shuttle, just not in the way it was originally intended.