A train was going "super-fast" just before it crashed into another service parked at a suburban Philadelphia station, injuring dozens of people, a passenger has said.

Experts say they are unsure anything struck the windscreen of an Amtrak train minutes before a deadly derailment in Philadelphia, in another twist to the investigation as the national network restarted its service to New York.

A buggy carrying a 14-month-old girl rolled off a slanted train station platform and fell onto the tracks, but the girl’s mother leaped onto the tracks to rescue her with the help of other passengers, transit officials said.