Lord Attenborough's appearance as the Old Gentleman marks his first ever TV role after nearly 60 years as an actor.
She was joined by other members of the cast- including Lord Attenborough - at the London Transport Museum to promote the remake, which is being shown on ITV on 23 April.
The 47-year-old plays the mother of her original character, Roberta, who is played by 18-year-old Jemima Rooper.
She said returning to E Nesbitt's story was "like looking at a favourite photograph in an album".
Attenborough's TV debut
"I first heard that Carlton was making a film of the railway children in a press story which stated I was going to play the part of the mother," she explained.
"That was before any casting had been done, but I would have been pretty miffed if I hadn't been asked after that."
Lord Attenborough's appearance as the Old Gentleman marks his first ever TV role after nearly 60 years as an actor.
He told how he was lured out of retirement as an actor at the age of 76 to take part in the production, written by Men Behaving Badly creator Simon Nye.
"Had it been a literal copy of the original I would not have wanted to take part, because I think the original has its own intrinsic qualities," he said.
"But this is a very different piece. It's much closer to the book and had an almost documentary feeling to it."
Family favourite
E Nesbitt's story, first published in 1906, centres on a family who go to live near a railway line when their father is wrongly branded as a spy and jailed.
The original Railway Children - filmed on the preserved Keighley and Worth Valley Railway in Yorkshire - became a family favourite soon after its release in 1970.
Along with Agutter, it also featured Sally Thomsett, Dinah Sheridan, Iain Cuthbertson and Bernard Cribbins.
The new production was filmed on the Bluebell Railway in Sussex.
Other stars include 13-year-old Jack Blumenau as Peter and Rab C Nesbit actor Gregor Fisher as Perks.
The Railway Children will be shown on ITV at 2000 BST on 23 April.