What is the connotation of the "65435" working method of urban rail transit?

Safety, accidents, dangers and hidden dangers. Urban rail transit is the backbone of urban public transportation, which has the characteristics of energy saving, land saving, large capacity, all-weather, no pollution (or less pollution) and safety. It is a green transportation system, especially suitable for large and medium-sized cities. The "six, five, four, three and five" working methods of urban rail transit are four connotations of safety, accident, danger and hidden danger.