Thousands of commuters got caught in rush hour delays when father's rights campaigners scaled a signal gantry over a busy rail line, a court heard.
The court heard the protest brought long delays
They dressed as Father Christmas for the protest, between Waterloo and Clapham Junction, on 16 December 2004.
The prosecution says many services were halted because of safety fears.
Gary Hollington and Helena Tilbury from Essex and Martin Mathews from London deny charges relating to safety and obstructing the railway.
Edward Gorecki, 48, of Dagenham, Essex, who is accused of staying on the ground to organise media interviews, also denies the charges at Middlesex Guildhall Crown Court.
Prosecutor David Durose said the four-hour protest, which began at 0745 BST, created a "significant backlog" of trains which caused delays throughout the day.
Rail bosses were concerned that the signals might have been interfered with, or might have been obstructed by the banner unfurled by the protesters, the court heard.
Eventually a Network Rail official confirmed the signals were visible and working, Mr Durose told the court.
"It was decided it was possible and would not be too much of a risk to allow trains to proceed through the area, but, because of the initial delays, there was in fact a knock-on effect throughout the day," he said.
"Why did they do it? Well, they were protesting in support of Fathers 4 Justice," he told the jury.
"But the Crown says that in seeking publicity for their organisation they broke the law.
"The reasons why they may have broken the law are not relevant to your consideration as to whether they did so."
Mr Hollington, 39, of Belfairs Heath, Romford, Mr Mathews, 38, of Westfields, Barnes, and 19-year-old Helena Tilbury, of Abridge Way, Barking, deny endangering passengers' safety and obstructing "any engine or carriage using the railway."
The trial was adjourned until Tuesday.