A passerby found the girl, identified as Alyssa B., Monday afternoon, The Local.de reported.
Police said they found the suspect, identified as Maurice M., on a nearby train track and arrested him. He had a suicide note in his pocket, The Local.de said.
The pair met in a German Internet chat room, Jappy, in May.
During the weekend, Maurice, 20, traveled 372 miles to meet the girl in person for the first time, despite her parents' strong disapproval, The Local.de said.
Classmates of the victim said he had serious feelings for Alyssa, and that he had threatened to kill her if she did not agree to marry him.
Witnesses said they had seen the couple fighting in the woods before the attack occurred.