It would be very cool, Bre Pettis decided, to take his desktop manufacturing robot, his MakerBot, out of its big black case and plug it in right there in the Amtrak dining car. It would be, like, ’bots on a train.

I was at Boston’s South Station, when I realized I was standing next to an Internet famous person. I’d seen Mr. Pettis’s Web videos for Make, a magazine for digital do-it-yourselfers. He has a face you remember. Thick black glasses, unruly hair and sideburns. He and his business partner Zach Hoeken, in a porkpie hat, were going home to Brooklyn after showing their MakerBot  a 3-D printer  at M.I.T.

In the dining car, Mr. Pettis started connecting cables as he explained. A computer slices a digital image into thin layers, which a MakerBot reads and replicates. It squirts hot plastic, the kind Legos are made of, onto a platform that moves up, down and sideways, building a replica one squiggly layer at a time. It can model anything you want: gears, or a goblet. Big factories use similar machines to make prototypes. MakerBot is a little factory for your home. Bre and Zach have a company that sells them, $750 for the basic kit.

A guy walked by and ogled the box glowing with L.E.D.’s. “These aren’t the droids you’re looking for,” Mr. Pettis said. That struck him as funny. He decided to recreate the “Star Wars” scene for an iPhone photo. He had me gesture like Obi-Wan. “These aren’t the droids you’re looking for,” I said, realizing I was in the presence of a highly appealing geekiness.