“My mama always said, ‘Life was like a box of chocolates — you never know what you’re gonna get.’ ” Those immortal words from Tom Hanks’s Forrest Gump hold true for buskers, too. After all, any aspiring entertainer can prop open an instrument case on a street corner, play a few bars, and hope for the kindness of strangers. Every now and then, though, you come upon a ringer. Which is exactly what I found on a recent morning at South Station. Two of them, actually — both young men, one in a red shirt and dark jacket, the other in a striped sweat shirt. I believe they were each playing a violin, though I couldn’t ask them, because they had perched on the inbound Red Line platform and I was headed outbound. Their gorgeous harmonies, rhythmic bowing, and silky vibrato filled the subterranean space. Commuters, often unmoved by subway buskers, appeared transfixed. A couple of guys with hard faces watched intently. It seemed clear to everyone that this was no run-of-the-mill street performance. Were they hotshots from Berklee College of Music or something? Their identity remains a mystery. Their skill does not.