She started with mistakes, months of them.

One has a fireball scrawled and almost scraped into the paper. The marks are furious, and underneath them there's a failed drawing of two hands locked in a pinky swear. Ellen Forney felt good making this. She was also crying. Hands are hard to paint. And these would be blown up to the size of a billboard to make Forney's first piece of public art. To depict a living hand she could believe in, the hands would have to include as few lines as possible, like a poem that's all wrong unless it has exactly the right number of syllables.

Sometimes she used a pencil, sometimes a brush loaded with ink. She alternated between whole hand portraits and parts rendered over and over. The strokes slither and spoon each other as they try to become correct. Forney would finish a drawing, look at it long and hard, and make handwritten notes on it—this angle is off, this part of the line is too thick, this curve is too round, this shape too pointy—before diving back into the next blank page. She'd struggle through a meditation of repetition and memorize movements. Turn the paper. Pull her arm smoothly. Put it all together into one uninterrupted flow of ink.

Only Forney would know the right hands when she saw them; these weren't Dürer's perfect hands or da Vinci's, they were Forney's perfect hands.

And she got them.

Today, graphic simplicity can be all you notice in Forney's 40-foot-long pinky swear at the entrance to the light rail station that's due to open in March on Broadway and John Street. From a passing car, the mural looks like a giant doodle.

The palette is bold and clean: human hands the color of marshmallows outlined in fluid black against a lollipop-red background. They're playful, sexy, and charming, the pinkies interlocking and squeezing, the few tension lines in the wrists telling the whole story of the movement as the forearms pull away to tighten the pinky bond, whatever it means, whatever it stands for, whatever two people are promising each other as they do it.

"I knew I would be looking at this thumb for a long time," Forney said, craning her neck. We were standing at the mural wearing hard hats as the art installers finished their work screwing together the heavy panels and bolting and bracketing them to the walls last week. Each mural is a grid of panels, and each panel, made of glossy porcelain enamel baked onto steel, is about as tall as Forney.

The station doesn't open until March, but already you can see her finished murals here and a few blocks south near Seattle Central College. The second mural shows a pair of walking fingers 30 feet high, in the same spare style. In the basic sense, they're useful. I can hear it already: Meet you at the pinky swear. Nah—at walking fingers.

They get better and better under scrutiny. There are these beautiful, odd shapes in a thumb crease or at the base of a wrist. One of the lines that crosses one of the wrists looks for all the world like a mustache, but it is also exactly the right shape to indicate the tension of fingers pulling. Every line is necessary, each absence adds. An undifferentiated white shape somehow manages to be a knuckle. Forney chose to silk-screen the panels, so that in the natural light, the thick liquid edges of the black lines shine.

Hiring Forney for this job was a leap of faith that paid off. The second-largest piece of art she's ever made is a four-by-four-foot painting.

She almost didn't apply, and then she didn't win.

In 2007, Forney had been publishing comics for 15 years, her first in Ms. magazine. She'd put out two books of comics with Fantagraphics and was working on a third, and she'd illustrated the young-adult novel by Sherman Alexie that was about to become a cultural phenomenon, The Absolutely True Diary of a Part-Time Indian. For Seattle Erotic Art Festival that year, she created four-by-four-foot paintings of hands doing sexual gestures—explicitly but innocently, since they were only hands.

Barbara Luecke, Sound Transit art program manager, knew Forney was an art activist on Capitol Hill (where she's lived since 1989) and asked Forney to sit on the light rail station artist selection panel—unless Forney "wanted to throw her hat in the ring." Forney hadn't even thought she was eligible for lack of experience.

At the close of 2007, Sound Transit received more than 150 artist applications. Sound Transit doesn't ask for proposals, just a résumé, statement of interest and knowledge of the project, and a portfolio of images. But Forney included in her packet a picture she'd Photoshopped using the Pioneer Square Station tunnel, depicting a giant cutout of Janet Galore, a Seattle artist and tech designer, wearing a pair of work boots and coveralls (hanging tantalizingly open at the breasts) and holding a wrench, standing several stories tall so that the trains would run between her legs.

The committee picked somebody else.

Brooklyn artist Mike Ross's team had created a sculpture at Burning Man that year that everybody was talking about, made of vertically balancing two 18-wheeler tanker trucks so that they looked like a raging two-headed beast. (In the Capitol Hill station, Ross suspended two fighter jets, chopped into segments and painted pink with their noses nearly kissing. You'll see that in March.)

But there had been something "invigorating" in Forney's work, "this clear energy that was very exciting even then," Luecke told me. The committee asked Sound Transit to find more money and hire Forney. Ross got a $550,000 budget, Forney $150,000. (Janet Galore would not have fit in Capitol Hill's tunnel, it turns out, but I still hold that image in my mind longingly.)

Forney had never had a project manager, or a team of fabricators and installers, or a team at all. Over eight long years, she had to learn the demands of unprecedented scale on a line drawing, to learn a new medium for the sake of color durability, to transform into a professional project manager. (And, oh, Forney also put out a best-selling graphic memoir about her bipolar disorder, Marbles, in 2012).

"Back in 2008, I could only think of these murals as hand-painted, and I thought I might be able to paint them MYSELF," she wrote in an e-mail. "Me, on a ladder."

She kept saying she can't believe how much she's grown as an artist. But Forney also knew what she didn't need to learn. Behind too many works of banal, unremarkable, failed public art there's a good artist and bad compromises. Forney held firm to her instincts about line, shape, color, and texture. This is her neighborhood, and she does care how the art comes across to the 14,000 other people projected to pass through this station daily by the year 2030. But Forney knew well enough to satisfy herself first.