The witness, who was interviewed by police and school officials, is now in counseling, said Norwood Borough Police Chief Mark DelVeccio.
DelVeccio said that the conductor, a trainee on the Acela, saw the two girls hugging when they were struck.
Dorwart had exchanged about eight text messages back and forth with the witness, asking the girls "to wait for her," said DelVeccio. "She wanted to join in to kill herself, too."
The last suicide by train in Norwood was last October, according to DelVeccio.
Friends of Dorwart and Gentile, both of Glenolden, said that the girls were upset about the death of a friend and classmate, William Bradley V, killed by a car while riding his bicycle to Gentile's home last month.
Regarding rumors of other suicide pacts, DelVecchio said, "There's nothing to indicate that's true, and the school has counselors on hand, so if any of their friends have a hard time, help is available."
Six days shy of her 16th birthday, the blue-eyed, brown-haired Dorwart had wanted friends to wear black and white outfits to her birthday party on Wednesday.
Since Thursday's tragedy, 4,600 visitors have posted messages of sympathy for the teens on Facebook.
On Friday night, a spontaneous vigil was held for the girls at the Norwood train station.
The two friends will be laid to rest next weekend.
A viewing for Vanessa Dorwart will be Friday, from 5 to 9 p.m. at the McCausland-Garrity Funeral Home, 202 E. Chester Pike, Glenolden. A Funeral Mass will be celebrated at 11 a.m. Saturday at St. Gabriel Church, 233 Mohawk Ave., Norwood. Interment will follow at Ss. Peter and Paul Cemetery, Marple Township. Contributions may be made to the Vanessa M. Dorwart Memorial Fund, c/o Beneficial Bank, 1862 Delmar Drive, Folcroft, PA 19032.
The viewing for Gina Gentile, from 5 to 9 p.m. Sunday, will be handled by Kevin M. Lyons Funeral Services, also at 202 E. Chester Pike. Both funeral directors provide services in the same building. A Funeral Mass will be celebrated at 11 a.m. Monday, also at St. Gabriel's. Interment is private.
A benefit for the Dorwart and Gentile families is planned for 2 p.m. Saturday at The Deck at Harbour Pointe, formerly The Lagoon, in Essington, Delaware County.
Meantime, Bucks County Medical Examiner Dr. Joseph Campbell is investigating an unrelated possible suicide on Saturday.
A 45-year-old woman was walking on the tracks when she was hit by the R7 SEPTA train enroute to Trenton at 5:30 p.m. just south of the Croydon Station.
"We're not sure if it was intentional or a tragic accident," said SEPTA spokesman Andrew Busch.
"There was no foul play."