Public transit exists, of course, to move people efficiently from Point A to Point B. But, hey, if riders can be sold something along the way, that’s good too, right?

That’s the thinking of some companies that are starting to look at transit patrons as perhaps the ultimate captive audience. One company that’s jumping on the mobile marketing idea is ING Direct, a financial services firm. Its pitch: It offers to buy one morning’s free ride for everyone using a big city’s rail system in return for being allowed to market its services in the stations.

Two systems, Boston’s T and the San Francisco area’s BART, have already given their patrons a free ride. Next up: Washington, D.C.’s Metro. This is an expensive promotion. ING Direct is offering to pay $600,000 for the Washington event. (ING wants a Thursday morning in December. On an average Thursday morning, about 230,000 riders use Metro’s rail system, although Metro officials figure others will show up because it’s free.) In return, the company wants to drape its signs on the turnstiles and have ING representatives on hand to pass out marketing information.

Sounds like a win-win-win: ING makes a splash, riders get a freebie, and Metro buys a little goodwill at no cost. But there’s business, and then there’s politics. And some on Metro’s board of directors were not pleased that ING wanted to offer free rides only to rail passengers, not bus riders. (Most Metro bus riders are in the city, while most rail passengers are suburbanites.)

Some city board members saw this in racial and economic terms, with bus riders being slighted because, as one board member put it, “they’re not as high-tech, they’re not as accessible, they’re not as visible in as many ways as rail riders.” Suburban board members were exasperated by the objections, one saying the Metro board was “looking a gift horse in the mouth.”

Nevertheless, the board asked its managers to negotiate with ING for including bus riders, a gesture some thought would add about $100,000 to the promotion’s cost.

Footnote: So why would a financial services firm want to sell itself to transit riders? Because big-city transit passengers are an attractive audience, a company spokesperson said. “It’s people who have a propensity to save, who look for a good value,” she added.

Postscript: After the hoopla about bus riders was raised, ING agreed to foot the bill for their free ride, too. One Washington city council member who’s on the Metro board was delighted, saying, “Every opportunity we have to not treat our bus riders as second class is a good opportunity.” As it turned out, the additional cost was only about $50,000.