LONDON—Jonathan Dunne, a 42-year-old from Colorado, set off for Covent Garden tube station Friday with a posse of helpers, many of whom he had met through Facebook . After being publicly ridiculed in the worst possible way the first time around, he was more determined than ever to realize his dream.

Someday, he hopes, Londoners will talk to one another on the tube.

“It’s not just me, the lone wolf in the station,” he said, motioning to his supporters.“I’m expecting to change the culture of London.”

A week earlier, the American expat, a health-education worker, handed out a batch of 500 badges embossed with the words “Tube Chat?” at an East London subway station—a bid to help spark conversation among the Underground’s ultra-reserved passengers. Things didn’t go the way he hoped.

The idea was “I would go hand them out and people would think it was funny,” Mr. Dunne said. “No one thought it was funny.”