In fact, what you saw was one of the latest forms of advertising technology, which is slowly taking over one of transit riders' last refuges from commercial messages.

The technology, which comes from companies such as Canada's SideTrack and New York-based Submedia, is just what it sounds like: ads displayed on subway tunnel walls in nearly 10 cities worldwide promoting products from companies including Microsoft, Target, Coca-Cola, Reebok and Honda.

Last month, Bay Area Rapid Transit (BART), which serves the San Francisco Bay Area, began a one-month trial of SideTrack's technology. The SideTrack system currently works by installing a long series of still photographs in a subway tunnel and then illuminating the images with rapidly flashing spotlights as trains go by. The effect is much like watching a movie, or a children's flip book, in that what riders see is a 15-second multimedia message.

And to those in charge of some of the rail systems using the ads, they're working.

"It's everything and more that I wanted it to be," said Graeme Hay, the commercial manager for London's Heathrow Express, which connects the British capital to its main airport. "I've had customers who said to me, 'Wow, that's fantastic. What is it?'"

Michael Swistun, CEO of SideTrack, explained that his company's technology is designed to present riders with a 24- or 30-frame-per-second "movie," depending on the speed of the train.

The technology requires that trains pass the ads going at least 25 miles per hour. If they're going slower, the lights stay off and the tunnels stay dark.

"We're cognizant that we don't want to be doing something irritating to people," Swistun said, "and the advertisers want to make sure the ads are displayed at the speed they're designed to be displayed at."

In BART's case, one major element still to be evaluated in its trial of the technology is whether ads threaten public safety.

"We're basically looking to make sure that the system doesn't distract our train operators," said Linton Johnson, BART's chief spokesman. "(So far) there's been no outcry from the train operators. That said, maybe it's because they enjoy looking at it."

Rider response

According to Swistun, as well as Hay and Johnson, there has been little, if any, complaining from riders who have seen the new ads.

But some do question whether any surface is safe from being used to sell.

"It's fascinating and yet sort of Orwellian," said Michael Vavricek, a regular BART rider. "It confirms what some humans say about America: Everything is for sale."

Others say the ads themselves aren't so bad, but that the tunnels could also be used to showcase art.

"I think (it's) cool as a medium, but just balance it out everywhere," said Oakland, Calif., artist Kevin Byall. "I feel like I'm being bombarded everywhere (by ads). The subway tunnels, I kind of like them dark. But if we're going to do this, make them artful."

Byall pointed to a in which they attached a projector to the side of a subway car which then displayed images of swimming fishes and sharks on the tunnel.

To the transit operators, however, such concerns are unfounded.

"We did think about that very carefully," said Hay, "because we have quite a lot of advertising along the routes, in the stations. The thing that made me feel comfortable that we weren't going to go into the area of turning (riders) off is simply because of the quality and the novelty of the technology. The user experience is so cool and so innovative that the 15 seconds actually excites people."

More to the point, the ads are bringing in much-needed revenue with hardly any cost.

Swistun said SideTrack has advertisers paying $50,000 a month for ads in Boston, and while he wouldn't be specific, he said that between 25 to 40 percent of revenue goes to the rail agencies.

"For them," said Swistun, "it's an absolutely perfect way to get new revenue without having to raise fare prices."

For now, SideTrack's ads have relied on still photographs viewed as the train shoots past. And because of that, it is a bit of a challenge to install--since the work must be done in the hours when the subways are closed. Any changes to an ad campaign must be done piece by piece.

But Swistun said that SideTrack is about to roll out a new, digital system in which the still photos and flashing lights will be replaced by LED screens. That way, he said, ads can be cycled throughout the day and campaigns can come and go without having to send crews deep into the tunnels.

Hay said that Heathrow Express--which is still in its initial three-month trial period in a single tunnel--is getting ready to add a second. And that second installation will use the digital technology.

Whether still photos or digital screens, the ads are changing the dynamic of the dark tunnel and the break in between subway stations jam-packed with ads.

But Swistun said he doesn't think rail passengers mind that tunnels are no longer dark.

"We're covering concrete and wires," said Swistun. "Nobody has said they'd rather see the concrete and wires. People are looking out the window anyway to avoid eye contact."