Who's that girl?

Used to causing a commotion wherever she goes, here's a totally incognito Madonna -- sans makeup, boy toy, kids or fans -- on a rare foray in the subway, hanging with sweaty riders who all but ignored the dressed-down Material Girl on a schlep downtown.

Sipping on a San Pellegrino Limonata soda and ridiculously overdressed for the hellishly hot underground in a jacket, scarf and baseball cap, Madge was joined Tuesday by two men and a young woman who helped her blend in with the hoi polloi.

"I don't mind you taking my photo, but be discreet," she told a lensman who spotted the 52-year-old Material Girl at the F-train station beneath 57th Street and Sixth Avenue.

No warning needed.

At a train change in SoHo, one teenage boy sauntered up to a stranger on the platform and asked, "Who's that?"

Getting no answer, the teen then walked nearer to the star, stared, circled around to the back of her buff behind -- and then gave up, wandering away puzzled.
Madonna and her pals ended their ride at a downtown station, where they were met by the exercise-loving mom of three's beefy driver-bodyguard. They then stopped at a bodega to buy beverages, a source said.

"Oh, that was Madonna?" the store owner sputtered after the visitors left. "I thought it could have been her, but I wasn't sure."

Madonna reportedly has been shooting a film in London and Paris titled "W.E.," centering around the lives of the Duke and Duchess of Windsor, juxtaposed with a modern American couple.