SCORES OF FERVID, pleading letters flowed into the mailroom of WETA last summer after the public TV station announced it might have to pull the plug on a popular show called "Shining Time Station."

"Please don't do this to us," begged Jack O'Leary of Washington, who described his 3-year-old son, Gavin, as "devoted beyond description" to the weekly program. Donald Anderson of Winchester, Va., reported he was "shocked" to hear of the show's possible demise. He wrote, "Our 2-year-old daughter was almost in tears. As she says, she 'loves that Thomas.' "

"That Thomas" is Thomas the Tank Engine, a toy locomotive who is the star of "Shining Time Station," seen Sunday mornings at 9 and Mondays at 11 on Channel 26 and Sundays at 10 on Channel 32. Though he mostly resembles a train engine, Thomas has a face that can change expression. He is adorable, spunky, virtuous, diligent, occasionally mischievous and unfailingly entertaining -- at least to the 2- to 7-year-old set that forms the bulk of his audience.

Thomas also is the focus of a burgeoning cult.

In response to letters like those quoted above, Quality Family Entertainment, producer of "Shining Time Station," recently began work on a batch of 20 new episodes to add to the 20 originals that have drawn raves from public-TV audiences. The new ones will be ready to air this fall.

Meantime, zealous fans have books from Random House, video tapes from Strand VCI Entertainment, clothing from Dayton Hudson, toys from Ertl and Playskool, and all sorts of other paraphernalia to help them sustain the Thomas experience.

And sustain it they do. Strand VCI spokeswoman Cathy Mantegna says "Thomas the Tank Engine" videos have been the company's number-one seller since they were introduced last June. Four cassettes cumulatively have sold more than 250,000 copies in America, and a fifth one is due out in February.

Minneapolis-based Dayton Hudson Department Store Co. reports brisk sales of "Shining Time Station" apparel and merchandise at all 61 of its outlets.

Locally you can buy the merchandise at department stores and specialty retailers that carry kid stuff. (Don't look for the clothing, though; it's now sold only in the Midwest, at Dayton's, Hudson's or Marshall Field stores.) Video stores and many bookstores stock "Thomas the Tank Engine" videos for rent and/or sale. And most booksellers with children's sections carry the popular Random House series.

It's not always pleasant to watch a kid-culture character rising to stardom in the United States. As the people who own rights to the character's image make one licensing deal after another, the appealing hero can become cloying, then grating, then obnoxious.

Even the good-hearted Thomas, through overexposure, may someday irk the people he presently charms. For now, though, he's still winsome and fresh, and so is "Shining Time Station."

If your kids haven't yet seen the show, you should urge them to take a look. Watch it with them: You're apt to get a kick out of it, too.

One major draw is Ringo Starr, who plays an 18-inch-high, magical character named Mr. Conductor. The rest of the regular troupe includes a female stationmaster, a lovable rogue, a train engineer and children Matt and Tanya. The half-hour show is structured as a sitcom, with a beginning, a middle, an end and an overarching lesson for young viewers.

Besides the real-people interaction, the show features puppetry, music videos and the all-important fables featuring Thomas the Tank Engine and his friends (most of them locomotives), who inhabit the fictional island of Sodor. Ringo narrates the fables, doing all the voices and many sound effects himself.

Each "Shining Time Station" episode features two fables, which run five-and-a-half minutes apiece and relate to the lesson at hand. For example, in one segment Thomas has an argument with a tractor because they're different. At the end of it, the tractor pulls Thomas out of a snowdrift and Thomas comes to appreciate the tractor's special qualities. In the show containing that segment, Matt and Tanya squabble because they have different interests. The theme echoes through the show, and the kids' little conflict is neatly resolved at the end.

A blandly wholesome formula, right?

Absolutely. And tykes love it.

Wesley H. Schmidt of Alexandria wrote to WETA that the show "has all of the elements which appeal to children without the violence, gaudy pictures and screaming and shouting which characterize a great number of the 'shows for children.' "

Comments like that kindle a rhapsodic glow in the soul of Britt Allcroft, the British TV producer and writer who brought Thomas to life in the electronic media.

In 1980, while working on a film about steam-powered trains, Allcroft rediscovered a group of children's stories she had enjoyed as a child. Collectively known to Britons as The Railway Series, the 104 stories -- featuring Thomas the Tank Engine -- were penned by the Rev. Wilbert Awdry in the mid-1940s through the mid-'70s.

Allcroft says that upon rereading them she sensed they would stand up well in a TV format. It took her three years to line up a production team and capture bank funding for the project.

Signing Ringo as the storyteller now seems a brilliant stroke, but it involved as much serendipity as sagacity. While she was pondering candidates for the role, Allcroft happened to hear Ringo being interviewed on the telly as she walked past a room in her home. Eureka. She wrote to the ex-Beatle, he called her, they met, she made her pitch and he agreed to give her his time and voice for a reasonable fee.

The first "Thomas the Tank Engine" episode aired on British TV in 1984. Though as pedagogic and chaste as Awdry's books, the show was "an instant hit," Allcroft says, "not just with kids but with the whole family."

As the show's initial 52 five-and-a-half-minute installments played and replayed, Thomas-inspired products began to materialize, everything from toys to stationery to fruit drinks.

Speaking by phone from London, Allcroft says around 100 companies now are licensed to market 500 products in England. With obvious pride she notes that the characters have become "part of the furniture of life" in that country.

To make the jump to American TV, Allcroft says she knew she would have to devise a half-hour format that would incorporate the short fables. Around the time she started considering such a venture, producer-writer Rick Siggelkow, who had seen a "Thomas" tape, called her from New York to propose the same idea. Allcroft and Siggelkow formed Quality Family Entertainment, a subsidiary of Allcroft's British production company, and together they created "Shining Time Station." PBS's New York City affiliate, WNET, co-produced the show in its first season.

The program, which premiered on virtually all PBS stations in January 1988, averaged a .9 rating nationwide in the 1989-90 season, which translates to 1.2 million viewers. Because PBS's contract with the producer allows the episodes to be shown only a maximum of four times in three years, many affiliates, including WETA, have urged "Shining Time Station" fans to rave about the show in writing and thereby encourage the making of more episodes. With 20 new ones on the way, the campaign paid off.

Allcroft says she hopes to make at least 65 episodes, which ultimately would enable "Shining Time Station" to run daily . . . and maybe to become part of the furniture of American life. If she succeeds, kids everywhere are in for a good time. This show is the kind of furniture they can climb all over without getting hurt.