CHICAGO — You wouldn’t think picking up milk at the “L” station would be a good idea.

But online grocer Peapod has turned a busy Chicago train station into a virtual supermarket aisle, enabling commuters to use their smartphones to scan and buy any of 70 items on the fly.

Appearing overnight on once-barren walls, seven-foot tall virtual shelves line both sides of a 60-foot tunnel, filled with everything from paper towels and diapers to fresh produce. Android and iPhone users can download a free Peapod mobile app to load up their electronic grocery carts for delivery the next day.

Chicago, Peapod’s home base and its largest market, is the second U.S. city to roll out the interactive supermarket shelves, which first appeared at Philadelphia train stations last month. Other Chicago locations may be added, and other cities down the line, depending on what happens during a 12-week run at the station, which averages 17,640 commuters — and potential shoppers — each weekday.

“It kind of changes the game for the out-of-home advertising medium, almost as a kind of service rather than branding,” said Dave Etherington, senior vice president of marketing and mobile for Titan, a New York-based media firm specializing in out-of-home advertising, which created the campaign for Peapod.

A combination of advertising and retailing, the first virtual supermarket was rolled out last summer by British-based chain Tesco, which set up shop at subway stations in Seoul, South Korea.

Skokie, Ill.-based Peapod took the idea as a promotional launching pad for its recent expansion into the Philadelphia market. In April, the interactive billboards were placed at 15 train station platforms across the city. The displays were winnowed down to nine locations, which will be running through early June.

Response to the ads has been strong, according to Etherington, both in terms of introducing the service, and selling the items on-site to virtual shoppers waiting to catch a train.

“It does a fantastic job of branding, but in a very transparent, measurable way, it ships products and it generates revenue,” Etherington said.

Founded in Evanston in 1989, Peapod pioneered the concept of online grocery shopping, long before many people even knew what the internet was. Perhaps ahead of its time, the publicly-traded company struggled during the dotcom bust, and was acquired by Netherlands-based Royal Ahold in 2001.

Peapod serves 24 U.S. markets and has delivered 21 million grocery orders in just over two decades.

With Internet shopping now fully integrated into the retail landscape, the company introduced its first mobile app about a year ago, trying to get out in front of the next wave in marketing. Used primarily by existing customers to re-order items — called “trash scanning” — the mobile app has new utility with the virtual supermarket billboards, both as a lure for new customers and a way for customers to fill out shopping lists on an ongoing basis.

“Grocery shopping doesn’t necessarily happen the way it used to,” said Mike Brennan, senior vice president and chief operating officer of Peapod. “It’s becoming more of a task that happens in multiple steps throughout the week.”

The shelves were fully stocked at State Street “L” last week as the first wave of commuters perused the virtual supermarket aisle on their way through the station. Some swiveled, others stay focused on the light at the end of the tunnel and a few stopped to shop, tossing their dog-eared shopping lists away for good.

Many commuters took notice of the very realistic depiction of typical grocery store shelves. Items included Huggies Diapers, Coke, Lysol, Swiffer Wet Jet, Bounty paper towels, Barilla ready-to-heat pasta and fresh produce such as apples and bananas. Like a real supermarket, shoppers may need to kneel down slightly to select items from the lower shelves, but most won’t need a stock boy to reach the top shelf with their smart phone scanner. Getting started is pretty easy, even for the technophobe, with instructions spelled out on the wall itself.

Making a purchase requires a smartphone and bar code scanner app. First, the shopper downloads a free Peapod app on the wall. To buy requires pointing at the item and scanning its barcode, which shows the price, then checking out. Items will show up at the customer’s door within a day.

Push the buy button and its loaded into a virtual shopping cart. Checkout with another click and the items will show up at your door within a day.

Virginia Marino, 28, an attorney, was headed to her office at noon after working from her Chicago home during the morning. She glanced from side to side with interest as she made her way down the tunnel, but questioned whether she would be able to shop during her normal commute.

“I typically take the ‘L’ during rush hour so I don’t know that I would take the time to really walk by with my phone and shop, because I’m trying to get home or I’m trying to go to the office,” Marino said. “And there’s no way you’re stopping because of the crowd — it’s just not going to happen.”

Brandon Precin, 23, of Evanston, Ill., who usually goes shopping only after he has run out of food, was impressed with the impulsive nature of virtual foraging.

“I think it’s really cool, being able to walk by, scan something and shop,” Precin said. “That’s a great concept. You get to look for what you want, scan with your phone and that’s it.”

Hurrying to class at Harold Washington College, Evelyn Ramirez, 20, of Chicago, slowed down enough to take in the new ambiance at the formerly pedestrian tunnel. It was, if nothing else, an artistic success.

“It’s kind of interesting,” Ramirez said. “I feel like I’m actually in a store right now.”

Peapod, which has relied mostly on direct marketing to draw customers, doesn’t expect its new campaign to become a primary retail channel for the company. It’s more about introducing its servive and its mobile app to new customers. While more locations may be in the offing, the logistics of a full virtual supermarket may be prohibitive, according to Brennan.

“There’s still only a limited number of products, no matter how many you put up on the billboard,” Brennan said. “Peapod has 12,000-plus products, and that would be a pretty long billboard to be walking down.”