A man was fatally struck by a train during the Monday morning rush in Williamsburg, cops said. 

The man, believed to be in his 40s, was struck by a J train at Marcy Avenue in Brooklyn around 9 a.m., cops said. 

The train’s emergency brake was activated when the man was hit, and the subway operator found his body on the tracks, police said. 

No criminality was suspected, but it’s unclear how the man ended up on the roadbed, police said.