Was it music? Was it intentional? Accidental?

Those were the questions from passengers lined up for Track 7 at lunchtime yesterday at 30th Street Station when New York City's Asphalt Orchestra launched a guerrilla-style debut in Philadelphia.

Cameras were fished out of bags. Eyes squinted toward the area between the Quik-Trak machines and the men's washroom as the intricacies of Frank Zappa's nervy and intense "Zomby Woof" bounced around the station's ultra-live acoustic.

"It's good, but, my God, it's overpowering," said one retirement-age woman from Connecticut (who asked not to be named) of the 12-piece new-music ensemble with distant roots in John Philip Sousa.

"My grandson plays trumpet, and if I ever catch him doing something like this, I'll wring his neck," said Dee from Langhorne - who didn't want her last name used - while boarding Amtrak's 12:42 p.m. train to Pittsburgh.

Philadelphia music professionals were tipped off to the performance via e-mail blasts. Scouts from the Mummers and the Kimmel Center showed up. Because the show was choreographed by the MacArthur fellow Susan Marshall, Dance USA director Lois Welk dropped everything and dashed to the train station, pronouncing the event "celebratory."

Drexel University's music program director, Mike Moss, zeroed in on the Balkan-flavored Goran Bregovic work, Champagne, as something for one of his wind ensembles. Composer Kile Smith's reaction was one word - "Fun!" - though he heard one passerby mutter a different single-word evaluation: "Stupid."

"Accidental audiences" - the word often used for those encountering high art in unlikely public places - often have see-no-evil, hear-no-evil reactions: If it can't be understood immediately, it doesn't exist. Those emerging from the train station elevator simply navigated past the musicians as if nothing were unusual.

Best to get used to the Asphalt Orchestra: The musicians aim to pop up most any place but conventional concert halls.

At the group's first performance, on Wednesday, it sprang from a subway entrance as part of Lincoln Center's Out of Doors Festival, which presents the Asphaltians through Aug. 23 in repertoire, including newly commissioned music as well as transcriptions of Bjork's Hyperballad and Charles Mingus' The Shoes of the Fisherman's Wife Are Some Jive Ass Slippers.

Yesterday's Philadelphia performance grew out of Lincoln Center's partnership with Amtrak; the idea was to show up at 30th Street Station at lunchtime for an hour or so. The orchestra's plans extend into 2010.

The idea grew out of Bang on a Can, the cutting-edge New York City collective of composers and musicians, founded by the composers David Lang, Michael Gordon, and the Philadelphia-born Julia Wolfe (whose mother drove in from Blue Bell to hear the band yesterday).

The group's notoriety stems from outlandish, improbable works for lineups such as 200 guitars or 100 tubas. A piece with the latter instrumentation prompted discussions of an avant-garde marching band. Rehearsal began in April.

The co-directors represent the project's yin and yang: Tall, blond Jessica Schmitz, 26, plays piccolo and has reasonably fond memories of playing in high school bands in the suburbs of Chicago, while the intense saxophonist Ken Thomson, 33, was the American Legion band's youngest member in Westchester County, N.Y., and recalls "terrible sound, terrible music, and people with whom I didn't feel kinship." That's why Asphalt Orchestra seems to simultaneously embrace and flee from the great American marching band tradition.

The 12-piece group, whose instrumentation includes sousaphone and two percussionists, has uniforms that aren't uniform. As designed by Elizabeth Hope Clancy (of the Broadway show Passing Strange), pants often show bare legs and shoulders often have military-style epaulets. Shoes range from tennies to Doc Martens.

"You're taking what's the most populist musical form, the marching band ... and sort of ripping it apart with music by iconoclastic composers that represent a sense of adventure ... and playfulness," said Bill Bragin, the Lincoln Center Out of Doors artistic director.

Possibilities are dizzying, such as Wolfe's new site-specific piece for Bordeaux, France, that was to have band members in individual open-air bicycle taxis converging from various parts of the city into a group of 150 student-age bell-ringers. The dates, however, didn't work out for the Asphaltians.

Thomson believes the future lies in art installations and indie movies, though at the Philadelphia show, one Mummers scout (who wished to remain anonymous) exclaimed, "We can use you!"