Complex sentence: He became a Pullman porter on the major railroads, at that time considered prestigious employment for African Americans, because it was relatively stable and well-paid, secure and gave freedom of travel and acquaintance.
Split into simpler sentences: He became a Pullman porter on the major railroads. At that time, Pullman porters were considered prestigious jobs for African Americans, as they were relatively stable and well-paid, secure and gave freedom of travel and acquaintance.