Laura Wilkinson, from Thamesmead, south-east London, said the two men, who "looked a bit druggy", donned balaclavas after boarding Tuesday's 8.12pm from Charing Cross to Dartford at London Bridge, and ran through the train demanding money.
Mrs Wilkinson, a mother-of-two, said the robbery happened so quickly many people did not know what was going on. "I could see them coming down the carriage towards me but I just sunk down into my chair. Suddenly the train stopped at Deptford and they jumped off and ran. No one had time to do anything. It was terrifying."
British Transport Police are investigating.