REQUEST OF EXPLANATIONS ABOUT WORSD AND PHRASES

Could you please explain me in details what was in the times of Martin Luther King the association "BROTHERHOOD OF SLEEPING CAR PORTERS"? What does it mean "PORTER" in this case? I have another question, that is what was a" BLACK TRADE UNION" in America? I found these words and phrases indicated in the context below: "They were Clifford and Virginia Durr, and Edgar D. Nixon Mr. and Mrs. Durr were white. He was a lawyer, and they were both active members of the civil rights movement. E.D. Nixon, a black man, was regional director of the BROTHERHOOD OF SLEEPING CAR PORTERS, the first BLACK TRADE UNION in America." Unfortunately I'm Italian and I strive to improve American English and American culture. Thank you. GIULIANO