The Call The Midwife star, who as a teenager shot to fame as the oldest of the Waterbury children in the 1970 film, has reprised her role as Bobbie, who is now a grandmother in the sequel.
The original film, based on the 1906 novel of the same name by Edith Nesbit, told the story of a mother and her three children who move to the Yorkshire countryside after their father is falsely imprisoned.
The sequel will see Smith play Agutter’s daughter Annie as the pair host refugees from Manchester evacuated to a Yorkshire village during the Second World War.
Speaking about working with Smith, 69-year-old Agutter told the Radio Times: “She’s an extraordinary, emotional, vulnerable, generous person and actor. You get the whole of her, always.
“That’s amazing to work with and lovely to be around. She hurls herself into it.”
The pair joined stars including Sir Tom Courtenay at the film’s world premiere at Keighley Picture House Cinema in West Yorkshire on Sunday after boarding a train at Oakworth Station in Haworth.
Agutter said taking the film into a different time is something author Nesbit would have liked, and she has been involved since the draft scripts.
“We felt that making this new piece, Nesbit could bring her story into another period.
“To play Roberta as a grandmother made me think about change in my own life, and how as we grow older we all live with it.
“I loved the idea of handing the baton on to a young person but showing at the same time that Bobbie has lived a life,” she said.
The new generation of The Railway Children includes The Crown actress Beau Gadsdon, as well as KJ Aikens, Eden Hamilton, Austin Haynes and Zac Cudby.
Gadsdon, who plays outspoken evacuee Lily, said she “screamed” when she found out she had got the role and is “excited to be part of the legacy”.
She added: “The first time I met Jenny was at Oakworth Station, it was magical.
“She brought so much colour to set and was so helpful, acting as a mentor. She helped us run through scenes and made sure we were comfortable.
“She also gave us advice and it felt like she was passing down (Railway Children) wisdom.
“The Railway Children Return carries the same spirit as the original — but it’s a different storyline.
“It carries messages of hope, resilience, family and how you can find human connection even in the most terrible of situations.”
The film is directed by Morgan Matthews and has been shot in key locations from the original film, including Oakworth Station and the nearby Bronte Parsonage.