Albert Whitman & Company announced on Tuesday that it has signed Newbery Medalist Patricia MacLachlan to write a prequel to Gertrude Chandler Warner’s The Boxcar Children series, which has sold more than 50 million copies since its debut in 1942. The novel will be released in September 2012 and will be published simultaneously as an e-book by Open Road Integrated Media. The announcement was made at the Gertrude Chandler Warner Boxcar Children Museum in the late author’s hometown of Putnam, Conn. Among those attending the event were MacLachlan, her agent Ruben Pfeffer, Albert Whitman senior editor Wendy McClure, and several former students of Warner, an elementary-school teacher who died in 1979.

McClure, who has overseen the series editorially since 1998, calls the news “very exciting.” Over the years, she has worked on developing what she terms a “bible” for the series, which is now written by “a small, select stable of ghost writers.” The publisher annually adds four novels to this series about four big-hearted and clever orphaned siblings. Warner authored the first 19 of the 150 installments currently in print.

“We wanted to return to the world of the first Boxcar Children novel and explore what it is that makes the kids who they are, which has piqued the curiosity of a lot of readers over the years,” says McClure of the decision to publish a prequel. The decision to tap MacLachlan for the authorial task was easy, she explains. “We thought of Patricia right away, since she writes in such a thoughtful, insightful way about families—and about families coming together in unusual ways.”

MacLachlan is thrilled with her new assignment. “It is kind of daunting, but of course very exciting at the same time,” she says of the project. “Daunting because it is such a very popular series. I’ve read a number of these novels, and the siblings are resourceful, independent, and most of all, incredibly kind to one another. I really like these children, and for the prequel I’ve tried to think of what kind of parents they must have had. They have such a strong sense of family—somebody clearly gave them that.”

After some fits and starts, the author has already begun writing the still untitled prequel, which she notes will include Depression-era historical details. “I agonized about the book during 7000 games of solitaire on my computer, and thought about it at midnight and five in the morning, but it’s finally starting to take shape,” she reports. “My granddaughter is four and her parents are reading Boxcar Children novels to her, and I’m sitting in on that, to see the reaction of a child to the books.”

After writing her first chapter, MacLachlan says she went back and reread some of Warner’s early novels. “What I am writing is my voice—but I don’t want to stray too far from hers,” she notes. “These are, after all her characters, and in a strange way I feel like I’m doing this for her. And that makes it all the more daunting!”