An expressman refers to anyone who has the duty of packing, managing, and ensuring the delivery of any cargo on board a train.
During the 19th century, an expressman was someone whose responsibility it was to ensure the safe delivery of a train's gold or currency, which was secured in the "express car". This job included guarding the safe or other strongboxes or coffers against outlaws, and memorizing the safe's combination until delivery.
Expressmen make an appearance in Mark Twain's short tale, "The Invalid's Story."