By Ellen Wulfhorst

NEW YORK (Reuters) - Funeral services were set for Monday for three victims of the deadly Philadelphia train derailment last week, as Amtrak commuter service resumed on the busy Northeast Corridor.

An investigation continued into the cause of the derailment that killed eight people and injured more than 200 others last Tuesday evening.

The train was barreling north at twice the 50-mile-per-hour speed limit when it entered a sharp curve and derailed just north of Philadelphia. It was headed to New York from Washington with 243 people on board.

The first Amtrak trains to travel that route since the derailment left early Monday from Philadelphia and from New York City.

Suspension of train service last week sent commuters scrambling for alternatives on the Northeast Corridor, the nation's busiest passenger rail line. More than 750,000 passenger trips are taken daily on the stretch from Washington through Baltimore, Philadelphia and New York to Boston.

A National Transportation Safety Board (NTSB) investigation is seeking to explain why the train accelerated to 106 mph from 70 mph in the minute before it derailed.

Investigators are looking into the possibility that a projectile struck the train after they found a circular pattern of damage on the locomotive's windshield after the accident.

The NTSB has not ruled out mechanical problems, human error or a deliberate act by the engineer, Brandon Bostian, 32.

The engineer, who suffered a concussion, has told investigators he has no memory of what occurred after the train pulled out of the North Philadelphia station, just before the crash.

NTSB member Robert Sumwalt said on Sunday that if the train was operating properly, it would have taken a deliberate move by the engineer for the train to gain speed.

"The only way that an operable train can accelerate would be if the engineer pushed the throttle forward," Sumwalt said on CNN.

The funeral for 39-year-old Rachel Jacobs, chief executive officer of a software start-up and mother of a 2-year-old daughter, was set to be held in Southfield, Michigan, according to local media.

Services were planned for Laura Finamore, 47, a real estate executive, in New York, and for Bob Gildersleeve, 45, of Maryland, in Holmdel, New Jersey.

Also killed were Justin Zemser, a U.S. Naval Academy midshipman; Jim Gaines, a video software architect at the Associated Press; Abid Gilani, a senior vice president at Wells Fargo; Derrick Griffith, a dean at Medgar Evers College in Brooklyn, New York, and Giuseppe Piras, 41, an executive from Italy.

(Editing by Jeffrey Benkoe)