Olympic and Titanic, both Royal Mail Ships.
Royal Mail Ships or sometimes called Royal Mail Steamers, are transatlantic liners who carried post over the Atlantic Ocean. Olympic, Titanic, Britannic, Lusitania, Mauretania, Aquitania and Carpathia were all Royal Mail Ships. The abbreviation for the Royal Mail Ships was RMS, which is still used by ships at the beginning of their names. The meaning was that they were contracted by the Post Companies to have large amounts of their mail in their cargo.
Postal workers on board the ships were hired separately from the rest of the ship's crew and were responsible for sorting mail en route to America as well as on the voyage back to the United Kingdom.