Hovering time train is pretty legitimate, but statistically, phone booths represent the most viable option; look at time travelers in popular culture. All of the most effective, longest-lasting time machines are based on phone booths. They're discrete, compact, and well-equipped for the job. You can even use most of the hardware already present. Even Doc Brown's DeLorean time machine uses a telephone keypad to punch in the destination date. Superman uses the time-manipulation properties of phone booths in order to travel through time so that he can always arrive to save the day in a punctual manner. Ever noticed how he's not only never too late, but also never too early? And he even has time to change into a tight spandex suit and slick back his hair. He also stows his briefcase, glasses, and suit in the fabric of spacetime so it doesn't get stolen while he's out saving the day.
