Witnesses said the 23-year-old, identified as Ahmad Al-H., attacked several people with a knife "for no apparent reason" at the Bayerischer Platz station in southwestern Berlin on New Year's Eve, according to a police report.
But the Tagesspiegel newspaper reported that he had asked one 50-year-old German man his nationality and called him an "infidel" before stabbing him.
The man suffered a light injury but declined medical treatment, police said.
Other people attacked by the knife wielder including a mother with a young child escaped unharmed, according to Tagesspiegel.
Police said only that the attacker - who the paper reported was known to officers after being involved in several brawls - "seemed deranged" when they arrested him in response to emergency calls.
Following the arrest, officers brought the knifeman to a clinic and had him examined by a psychiatrist.
"We can't confirm or deny (the reports), we are checking whether there may be an Islamist background," a police spokesman told AFP.
The investigation has been passed to a unit which deals with politically motivated crimes, according to the police report.