Amtrak spokesman Marc Magliari said Sunday that the railroad is searching for 69-year-old Charlie Dowd.
Dowd, who lives in San Mateo, Calif., boarded the Amtrak train on Wednesday in the San Francisco area.
Dowd's son talked to him by cellphone on Thursday evening when the train was in the Denver area. That was the last time family members talked to Dowd.
Dowd's luggage, cellphone and medication arrived in Chicago without him on Friday. Family members say they are worried that Dowd may become disoriented without his medication.
Dowd is a retired firefighter who is 5 feet 10 inches tall and weighs 175 pounds.