A man was killed when he was struck by a subway train in Manhattan early Sunday, authorities said.

The man, believed to be in his 20s, was walking in the tunnel between Union Square and the 23rd St. station on the Q line when he was struck by a northbound train around 4:30 a.m.

The train operator did not see the man on the tracks but knew something was wrong when the train’s emergency brake was activated.

When the operator got out to investigate, he discovered the man lying on the tracks underneath the train and called 911.

The man’s identity has not been released and it is unclear why he was on the tracks.