Britt Allcroft, a British producer, director and writer who adapted the antics of a cheerful, impetuous locomotive into a TV series, “Thomas & Friends,” that became a sprawling franchise and a longtime favorite of children and adults, died on Dec. 25 in Los Angeles. She was 81.

Her death was confirmed by her daughter Holly Wright.

Ms. Allcroft was the driving force behind bringing Thomas the Tank Engine, an animated locomotive first conceived in a series of children’s books in the 1940s by Rev. Wilbert Awdry, to television screens. The series spawned a movie, merchandise and even theme parks, ballooning into a billion-dollar franchise.

Her original adaptation was a live-action production with a small budget that premiered in 1984 on the British channel ITV as “Thomas the Tank Engine & Friends,” later shortened to “Thomas & Friends.” Audiences were quickly drawn to the show’s life lessons packaged with catchy music, lush landscapes and an entertaining ensemble cast of humanlike locomotives. It became a runaway success that aired for more than three decades.

“Children live these days in a fast-paced world, but I don’t think children really change,” Ms. Allcroft said in a 1995 BBC documentary. “They need in their lives gentleness, comfort. They need fun, and they need stories that, whilst being entertaining for them, also help them interpret the world.”

Ms. Allcroft first encountered the series while researching a documentary on railroads. Earlier attempts to adapt the books to TV had faltered, but she pushed for the rights, envisioning a series with a narrator who hewed closely to the original books.

Her conviction led her to finance much of it herself, even mortgaging her house, she said in an 1995 interview with Australia’s “60 Minutes.”