Mr Etienne said the man walked further down the train before it went "eerily quiet".
"Then there was a scream and about three or four men jumped towards the scream. At that point everyone in the front two carriages moved with haste towards my car," he said.
When the train reached Forest Hill, Mr Etienne said he saw a woman in a hijab come out onto the platform "wailing" and with "blood on her hands".
He added that the man was now on the platform, shouting: "I am the anti-Christ, you don't have to worry, I am going to kill me a Muslim."
Witnesses described the "horrific" attack and its aftermath on Twitter.
"The perpetrator is currently ranting on the streets. Beware," one wrote.
Another tweeted: "#foresthill The attacker shouted "Who is Muslim, I want to kill a Muslim", approached the guy sitting with his wife on the 1st carriage."
Shellby Curry, 24, was with her one-year-old child when she saw a man "waving what looked to be a knife in his hand" near the station and screaming words like "Muslims f****** hate them, kill them all".
"I was walking along and someone stopped and said, 'Be careful, he's got a knife'.
"As I looked around there he was. I just grabbed my baby and ran across the road. Looking back (I saw) him marching up and down waving his arm about."