O'Sullivan said the train's guard came over the public address system asking people what they were doing on the tracks and warning them that they could die if they touched a rail.
The train had been traveling between Shepperton and London Waterloo when the man's Bible-reading led to a "commotion".
Fellow commuters in their panic prised open the train doors and climbed down onto the track to escape the man, forcing the rail operator to turn off power to the area to prevent anyone being electrocuted.
"Some guy tried to tell him to stop it as he was scaring people and causing a crush, that it sounded like he was going to end it. Power switched off now on train".
But other passengers said the man referred to "doomsday" and life after death, which spooked many people.
A spokesman for South Western Railway confirmed "there was an incident" this morning and said all trains are running again, with some delays expected.
"He recited lines about homosexuality and sex outside marriage being a sin, God gave his only son for our sins etc".
He said the guard came to the carriage, spoke to the now "sheepish" man and searched his bag, but found only a water flask and books.
BTP said it was still trying to establish what had happened but said no arrests had been made.
"British Transport Police are investigating and there were no injuries to passengers or staff". This was a packed passenger train at the height of the rush hour.
Mick Cash, leader of the RMT union, said: "Once again the guard has stepped in and calmed a potentially risky situation in an exemplary and professional manner".
Elsewhere on the SWR network, a swan on the line between Petersfield and Portsmouth caused delays for up to 40 minutes. "It illustrates just why RMT members are fighting so hard to keep the guard on South Western Railway services".
Southern, Gatwick Express and Thameslink services were also delayed because of overrunning engineering work.
SWR services had already been disrupted because of an earlier points problem affecting five platforms at Waterloo, just weeks after a multi-million pound improvement project was completed.
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