Dowd was last heard from via cell phone on Thursday and did not arrive in Chicago that day as scheduled, while his luggage, cell phone, and medication did.
A train conductor who spoke with the 69-year-old early Friday morning near Omaha, Nebraska reported that he found Dowd to be disoriented and confused about his whereabouts.
"He does, uh, has medical needs, and does need his medication for high blood pressure, for heart disease, and he is diabetic," daughter Jennifer Dowd said. "But, you know, for medical reasons, we obviously need to know where he is."
On Sunday Jennifer left for Omaha to distribute fliers and search for her father on the ground.
Dowd is 5 feet 10 inches tall and weighs about 175 pounds with blue eyes and gray hair, she said.
The family has created a Facebook page to help locate him.
Anyone with any information about Charles Dowd's whereabouts is asked to call Amtrak police at (800) 331-0008 or the Dowd family at (650) 766-6470.
(Bay City News contributed to this story)