Investigators were on the scene Friday night, hours after the accident, which happened in the midst of rush hour.
Lake Forest police say the two victims were walking northbound when the train, headed the same direction, hit them.
The accident happened about a one-quarter mile north of Conway Road in Lake Forest.
"The units came out here, and they found one teenage-age person who was alive and talking to us. And she told us that there was a second person with her. And we searched the tracks and the area, and eventually found that person, who was deceased," said Deputy Police Chief Dave Field.
The accident took place behind a residential area, where there was no easy access to the tracks. There are no roads or crossings in the immediate area.
In fact, Amtrak officials say the pedestrians were trespassing.
Amtrack officials also told ABC7 Chicago it was Hiawatha Train #339 headed from Chicago to Milwaukee that was involved in the accident.
Approximately 138 people were onboard. The crew members and passengers were not injured.
The male victim was pronounced dead at the scene, and the female was transported to advocate Condell Medical Center in Libertyville.
A hospital spokesperson told ABC7 that the female victim was in critical but stable condition.
The names of the victims were not being released Friday night, as police continued their investigation.