It’s a cold December evening. The sun slipped away long ago. Commuters — hurried, wrapped, gruff-looking — stream out of the turnstiles at the Green Street T stop. Then they hear the Christmas carols. “I didn’t think anybody did this anymore!” says a woman in jeans and a tan jacket. She stops, puts her bags down on a bench, and joins the 10 or so people from Hope Central Church, just up the street in Jamaica Plain, who have caroled at local T stations during the holidays for several years. The motley chorus performs “Silent Night,” “Away in a Manger,” and other seasonal anthems just inside the doors, their voices competing with announcements over the loudspeaker and the trains’ screeches and rumbles. Their cheerful presence proves infectious. Faces brighten. People linger to listen and sing. “And they shouted out with glee!” a father belts out as he stuffs his son’s hands into gloves. A woman taps her black shoe on the brick floor. A young couple walks out onto Green Street. “What is the song?” the woman asks. “Hark! The herald angels sing,” her companion croons in reply, his voice trailing off into the darkness.