NEWARK — “ Clear the floor,” said Nile Ahmid, a D.J. at the Branch Brook Roller Skating Center in New Jersey. It was around midnight on a recent Sunday, and he had been spinning house and hip-hop tracks for the past three hours. “The next skate is for trains only,” he added.

The lights came on, and 400 skaters brought their wheels to a halt. On cue, they linked arms in prearranged groups of three to 10 rollers to form so-called trains, a hallmark of the skating style specific to New York and New Jersey.

Rollers ranged in age from late teens to 60s, and were both gay and straight. But in terms of race, about 95 percent were African-American, a demographic that has both defined and given historical context to similar adult-night skating events in scores of cities nationwide.