The driver in Monday's accident may have fallen asleep at the wheel, Knott said.
The accident was discovered at 6:30 a.m. by a Hagerstown man who was walking along the railroad tracks near Lakeside Trailer Park.
"I saw something under the I-70 bridge," said Raymond Moats. "I walked up to it and saw two people on the ground about 10 feet from a white sedan that was upside down."
Moats, 36, of Highland Manor, said the 19-year-old woman was conscious and calling for help.
"She told me they were traveling from Pennsylvania and she was sleeping in the car," Moats said. "She said she'd been laying there for hours, but it probably wasn't that long."
Located in a remote area, the car couldn't be seen from the interstate, Moats said.
"About that time, a train was coming and I waved at the engineer and pointed toward the car and the people," Moats said.
He said the engineer acknowledged him and called Washington County Fire and Rescue Communications.
Earlier Monday, state police had received a call about a car going off the road, but the directions were faulty and no car was found during a 5 a.m. search, police said.
Police said the driver's family did not know the identity of the passenger in the car. Knott said police were having trouble locating her family.
"It's just a matter of time. It may take a little longer than normal," he said.
It was the fourth fatal traffic accident in Washington County this year, state police said.
Knott said the accident illustrates two important points - the need to keep alert on the road and the benefit of wearing seat belts.
The driver and passenger in Monday's accident, neither of whom were wearing seat belts, were thrown from the car during the crash.
While it is too early in the investigation to determine if seat belts would have made a difference, Knott said seat belts were not worn in two of the three other fatalities in the county.
"At least with the other two, they would probably still be alive," he said.
Knott said there have been two recent accidents on Interstate 70 in the western part of the county in which drivers also fell asleep.
"It's kind of desolate. There's not much out there," he said. "Folks are out there for long distances and they try to stretch it."
Drowsiness can lead to crashes, Knott said. He said drivers should pull off the highway and walk around and should check into a motel if that doesn't help them remain alert.
Upcoming engineering changes also may help.
The Maryland State Highway Administration now automatically installs rumble strips on the inside and outside median strips when roads are resurfaced.
Steve Davis, the resident engineer for Washington County, said three repavement projects under contract will include rumble strips, grooves in the asphalt that are higher than the roadway.
The locations are Black Rock Road to U.S. 40; just east of Md. 56 to just east of Boyd Road; and just east of Boyd Road to just west of Little Conococheague Creek.
"It's just an additional safety practice. Whether they fall asleep or inadvertently run off the road, it alerts them quick to get back on the road again," Davis said. "Unfortunately, it only takes a second with the speed limits that we have," he said.
Davis said the agency is putting together contracts now to install rumble strips on I-70 and I-81. They will be done in sections, he said.