But get your nose out of that book, shut that laptop, pay attention or you'll miss it as you scoot through the eastbound tunnel from Montgomery Street to Embarcadero.
If you do see it, you'll see Warhol-esque multiple-screen movies in red, with images of tiny Target department store logos raining onto an attractive young man and woman who appear to be taking showers, kayaking and springing on diving boards.
BART didn't need to turn its windows into a high-tech heads-up display to do this, however.
The high-tech part comes with sensors that illuminate the posters when a train approaches. The rest goes back to the well-worn idea that when one looks at pictures of the same thing in slightly different poses in quick succession, they appear to move.
"I was in Paris riding the subway for two weeks in a row, wondering what we could do with all that empty space," explained Rob Walker, chief executive officer of SideTrack Technologies Inc., a Winnipeg, Canada-based firm that began installing the "videowall" advertisements five years ago in Boston's subway.
So he collaborated with Bradley Caruk, who has worked on animation projects with film studios Disney and MGM, and created SideTrack, which has installed the ads in subways in Mexico City, Toronto and Rio de Janeiro and most recently in London's airport express train.
Although the idea came from children's books that have an image that moves when the reader flips quickly through the pages, it operates on the scientific principle of persistence of vision, Walker said as he chatted in the Embarcadero BART station with executives from CBS Outdoor, BART's advertising contractor.
"Above 20 images a second, the mind blends that into constant motion," Walker said. "A lot of people think they're TVs."
As Wednesday's testing commenced in the single BART tunnel, however, riders on several trips paid no attention to the moving pictures just outside the train windows.
"We're in a test period right now, so we're going to play with the light levels a little bit," Walker said.
BART spokesman Linton Johnson said the ad experiment should not interfere with the operation of the trains.
"It can only be a certain brightness for safety reasons," he said. "When the light comes on, we don't want it to glare back in the operator's eyes."
While several trips between the two stations came up with no riders who actually noticed the advertisement, BART rider Amanda Pruitt of Concord had something to say about them.
"I've been in advertising, and anything that's hidden like that is a waste of money," she said. "I just got off work, I'm tired. Very few people are going to look out that window for an ad."