PHILADELPHIA - Amtrak officials are trying to determine what kind of mysterious object struck one of its trains in Philadelphia.

The Acela Express was headed to New York City from Washington with 201 passengers aboard on Sunday night when the unknown object cracked a window. No one was injured.

The train stopped at Metropark in New Jersey so inspectors could check the damage.

The incident occurred near where an Amtrak train derailed in May, killing eight people and injuring more than 200.