The "R.R.B." printed on this sack is the abbreviation for Rail Road Business. Railroad business mail handled in the baggage car did not pass through U. S. Post Offices. Instead, it was dispatched solely between railroad offices. Only matter pertaining to railroad business transactions and operations was permitted; no personal correspondence was allowed because that was prohibited by the Private Express Statute.
The right of railroads to transport and distribute their own mail outside of the Post Office Department/Postal Service was provided an exemption from the Private Express Statute. The Private Express Statute of 1845 and the Postal Code of 1872 otherwise ceded exclusive rights for handling first and third-class mail to the federal government.
R.R.B. mail was handled in a passenger train's baggage car. The baggage car was often located behind express and mail cars and ahead of the passenger-carrying cars. The railroad employee in charge of the car was called the "baggageman" or "Baggage Master." In addition to looking after the passenger luggage and corpses, the baggageman also received, sorted and distributed the railroad company's business mail passing between railway officers and agents.