A Poughkeepsie mother was arrested after allegedly assaulting her child aboard a Metro-North train, authorities said.

Tatiana Lee Kraemer, 37, was taken into custody Sunday night after a train arrived at the Tarrytown station, according to MTA police.

A retired NYPD officer told police he witnessed Kraemer slap the 6-year-old girl in the face while aboard a train traveling from Grand Central to Poughkeepsie.

Officers observed the child bleeding from the mouth and arrested Kraemer, authorities said.

Kraemer was charged with assault and endangering the welfare of a child.

The girl was treated at the scene and released to another family member, police said.