Quiet, even on New Jersey Transit's new quiet cars, can be a relative term. In the chat-free coaches that made their debut Tuesday, creaking air-conditioning units and an incessant rumbling went unchallenged by the usual din of cell phone conversations, the de facto soundtrack of the daily commute. 'It is a movement toward civility,' said Frank Cioffi, an English professor who often finds himself infuriated at gabbing riders on his commutes to Princeton Junction. 'You've got these high-powered businessmen in the morning doing heavy-duty negotiations, while you're trying to read a book.' As part of a three-month pilot program, 29 express trains between Trenton and Manhattan have acquired two quiet cars, one at each end, that promise commuters a Zen-like cocoon: 'No cell, no song/low talk/in peace we travel/arrive calm,' read the pale-blue promotional posters. Of course, ornery commuters have been enforcing de facto quiet cars since the dawn of the mobile age. But if reviews are positive, the pilot program could expand. But how to ensure that quiet-car denizens stay quiet? 'Peer pressure,' said James Weinstein, executive director of the railroad. 'If somebody gets on and they're not aware it's a quiet car, and they start their cell phone, first they get killed by looks. And then if that doesn't work, passengers will invariably get up and let folks know that it's a quiet car and cell phones are not permitted there.' Such self-policing has been standard practice on Amtrak since the railroad pioneered the quiet car in 1999. (The amenity, now on most Amtrak trains in the Northeast, arose from a passenger's suggestion.) But there can be a fine line between enforcement and vigilantism. One maligned Amtrak rider, writing to The New York Times in 2004, suggested the railroad 'give train conductors guns' to contend with defiant 'cell-yellers.' This is not the strategy that New Jersey Transit is using. Its preferred method: a business card slipped surreptitiously to the offending yakker, encouraging a lowering of volume. But the railroad may need to mind its methods of chastisement. On Philadelphia's commuter trains, where quiet cars debuted in 2008, 'the crews had to be diplomatic,' said Kim Heinle, a customer service executive at the railroad. Some people, he said, 'didn't want to shut up.' 'You have to remind people that it's not against the law to talk; it's a social change,' Heinle said. 'But be sensitive to the fact that somebody might not know the rules.' New Jersey Transit is the nation's largest commuter railroad yet to receive quiet cars. (In addition to Philadelphia, the cars are used on rail networks in Maryland, Virginia and parts of California.) 