Genesis 4:18

“To Enoch was born Irad, and Irad was the father of Mehujael, and Mehujael was the father of Methushael, and Methushael was the father of Lamech.”  Genesis 4:18

So Cain became a grandfather and then a great grandfather and his line went on.  Despite his murder of his brother, God allowed him to have a family.  They had some interesting names.  Irad means "fugitive," Mehujael means "smitten of God," and Methushael means "man who is of God."  The meaning of Lamech is unknown.