The San Bernardino-bound train, which started its journey in downtown Los Angeles, was between the Upland and Rancho Cucamonga stations when it hit the person around 3:30 p.m., said San Bernardino County sheriff's spokesman Randy Naquin.
The person was a man who appeared to be in his 20s and wasn't carrying identification, San Bernardino County sheriff's spokeswoman Teresa McMahon said, adding that nobody on the train was injured.
Nearby, however, a cellphone was found containing what appeared to be a suicide text message, according to Lt. Richard Hoffman of the Los Angeles County sheriff's transit policing division.
In addition to the sheriff's departments, the Ontario police and the Rancho Cucamonga and Ontario fire departments responded to the scene, Naquin said.
The stretch of track was reopened about 6 p.m.
Metrolink could not be reached for comment.