The attacker is said to have stabbed fellow travelers in the ICE 928 completely suddenly and indiscriminately.
According to preliminary information from security circles on Sunday, the attack was not preceded by a dispute.
According to the information, the suspect - a 27-year-old who previously lived in Syria and came to Germany in 2014 - recently lost his job.
According to an initial assessment, the attacker may have suffered from mental health problems and delusions.
On Sunday morning there was still no reliable information about the motif.
During the search of the alleged attacker's home, nothing suspicious was immediately found.
The suspect is said to have given no information about the motive after his arrest.
An evaluation of the data from his cell phone is still pending.
The arrested person who was recognized as a refugee in Germany last lived in Passau.
In the Passau-Hamburg train, three men aged 26, 39 and 60 were seriously wounded on Saturday morning and were taken to hospitals.
According to the police, one of the victims was able to leave the clinic on Saturday evening.
A collapsible knife with a blade length of eight centimeters is said to have been seized as the murder weapon.
So far, nothing is known about a possible extremist background to the knife attack.