Seventy years after its debut, the Boxcar Children series is finally getting a prequel – and by a Newbery Medalist to boot. In The Boxcar Children Beginning: The Aldens of Fair Meadow Farm, Patricia MacLachlan, author of Sarah Plain and Tall, provides what she calls “the beginning before the beginning” of Gertrude Chandler Warner’s popular series about four siblings who live alone in a railroad car in the woods. Due September 1 from Albert Whitman, the novel offers a backstory to the series, which debuted in 1942 and has grown to more than 150 titles across a variety of formats, including beginning readers, graphic novels, and special editions. Warner, who thought up the plot of the original novel one day while she was home sick from her teaching job, clearly conceived of a winning premise: the Boxcar Children Mysteries currently has more than 50 million copies in print worldwide.

The original novel about the Alden children was published by Scott, Foresman and Company, and Albert Whitman soon thereafter became co-publisher, issuing trade editions of that book and subsequent Boxcar Children Mysteries, while Scott, Foresman published editions for the school market. In the 1960s, Albert Whitman acquired full rights to the series. Warner wrote 19 installments before her death in 1979, after which ghostwriters took over authorship.

Senior editor Wendy McClure has been at the editorial helm of the series for the past decade; she explains that the prequel was sparked in part by letters that the publisher has received from readers over the years. “There are a couple of questions that readers always ask,” she says. “One of them is, ‘What happened to the parents?’ We realized that there was a story there and decided that it was time it was told.”

MacLachlan was an easy choice, according to McClure. “We knew from the start she’d be perfect,” she says. “She writes so beautifully about families and about families coming together in unusual ways. She had great insight into the young characters.”

When approached, MacLachlan says, she willingly took on the task. “A door opened and I decided to walk through it and see how it goes,” she says. “I like these children a great deal, which made it easier to write about them.” MacLachlan says that she did find the prospect of writing a prequel a bit daunting, but it provided her with some boundaries, too. “I did invent the parents, but in a sense I knew what kind of parents they had been, because of who their children are. The siblings are thoughtful and kind, and they take care of each other.”

In preparation for writing The Boxcar Children Beginning, MacLachlan read a number of the series’ installments – and listened to her son read them to her six-year-old granddaughter. “I sat on the edge of her bed as he read the books, and I was able to hear her reaction to them,” she says. “This helped draw me into the lives of these characters. ”

The author also drew from her own grandparents to create the prequel, in which the Aldens share their farmhouse with another family that has lost its home in the Depression. “My grandparents took in a lot of people who had lost their jobs during [that time],” she says. “So I was able to use their lives as well.”

Warner’s former students also provided MacLachlan with inspiration. A group of them accompanied the author to the Gertrude Chandler Warner Boxcar Children Museum, located in a 1920s railroad boxcar in Warner’s hometown of Putnam, Conn. “They spoke of their former teacher with such affection and humor that it made me feel even closer to her – and feel a responsibility to her, to them, and to the characters she invented. I feel as though we all kind of share their story.”

MacLachlan will return to the museum on October 20. That is the final day of her eight-city tour promoting The Boxcar Children Beginning and the series’ 70th anniversary, which kicks off in Boston on September 28. As part of the commemoration, Albert Whitman is also reissuing the earlier novels with new cover art by Tim Jessell.

Though the writing of the book is behind her, MacLachlan says the Alden children haven’t entirely disappeared from her life. “I sometimes dream of them going back to the farm,” she says. “I’m not exactly sure where that comes from. I guess they have in a sense become part of my family.”

The Boxcar Children Beginning: The Aldens of Fair Meadow Farm by Patricia MacLachlan, based on the series by Gertrude Chandler Warner. Albert Whitman, $16.99 Sept. ISBN 978-0-8075-6616-9