A man evicted from a shelter was the attacker.
Four mothers and two children were among the immigrants meeting the Pope in Philadelphia.
"Those ringing words continue to inspire us today," he said of the Declaration of Independence.
The leader of the Roman Catholic Archdiocese of Philadelphia said the idea "plays on our worst fears and resentments."
More than 200 people were injured in the wreck that plunged screaming passengers into darkness and chaos Tuesday night.