“Two Trains Running,” the second offering in the Signature Theater Company’s August Wilson season, has sold out its originally skedded run and extended by a week — before the production has even begun previews.Starting perfs Nov. 7, “Two Trains” will now run through Jan. 7. Off Broadway’s Signature Theater had a hit earlier this season with its first Wilson show of its season, “Seven Guitars.” That production won strong notices and extended two weeks. “Two Trains” centers on the regulars of a Pittsburgh diner in 1969. Directed by Lou Bellamy, the cast includes Frankie Faison and Ron Cephas Jones. The Signature’s $15 ticket initiative, the result of a Time Warner sponsorship, applies only to the original run. Tickets to the extension will be $55.