The Metropolitan Transportation Authority said four or five carriages on the seven-carriages derailed close to the Spuyten Duyvil station in the Bronx, New York.
None of the cars entered the Hudson or Harlem rivers, which are adjacent.
Early reports indicate that four people have died and a further 48 are injured. This figure is expected to change.
More top news
Orlando Bloom has visited children whose school was destroyed by shellfire during the conflict in Ukraine.
Jeremy Corbyn has launched a fresh attack on "ideological" Conservative cuts ahead of Thursday's elections.
Daddy Day Care star Eddie Murphy is celebrating the birth of his ninth child.