BOSTON — Francis P. Salemme, 84, arrived in federal court this week in a wheelchair, his hunched frame shrouded in a loose suit and his feet tucked into easy black sneakers. He gripped the arms of a wooden chair behind the defense table, gingerly rising for the jury’s arrival. His slight face, papery skin and wispy gray hair were a startling departure from his mug shots of long ago, back when his jaw was set like concrete.

It can be hard to absorb that this frail man used to be known as Cadillac Frank, a fearsome gangster who admitted to multiple killings, went to prison for a car bombing that blew a man’s leg off, and survived an assassination attempt outside an International House of Pancakes. He was once a powerful mafia boss, the head of the New England family of La Cosa Nostra, the authorities say, and a contemporary of James “Whitey” Bulger, the notorious Boston crime boss.

Not long ago, human remains turned up behind a mill building in Providence, R.I., setting into motion this new murder trial against Mr. Salemme. But the authorities say the crime itself took place a quarter-century ago. And most everything feels like a flashback in the trial that began this week, including the who’s who of underworld players trudging into court in sensible shoes. It is a reminder that it has been a long time since a clear-cut set of larger-than-life gangsters controlled New England’s criminal underworld.

Anthony Cardinale, a defense lawyer who has represented mobsters — including, decades ago, Mr. Salemme — described the trial here as a “last vestige” of such federal prosecutions. “Everybody’s been burned to a crisp here by informants,” he said.