NEW YORK -- Every day, throngs of riders stand on the edge of danger in the nation's busiest subway system, waiting on platforms with nothing between them and the tracks.
Dozens of subway and light rail systems around the world have safety barriers with sliding doors on their platforms, but the idea hasn't gotten traction in New York. Yet transit officials are giving it a new look after two people were pushed and a third fell to their deaths on the tracks since early December.
Safety doors would be expensive and difficult additions to the sprawling, 108-year-old subway system, but some people are urging the Metropolitan Transportation Authority to try it. A company has proposed to install the barriers for free in exchange for advertising revenue.
Being shoved or bumped onto the tracks is "my biggest worry about New York," said Ed David, a cinematographer who last spring launched an online petition to install the devices after reading about a college student who was hurled onto the tracks and killed by a train amid a fight at a Brooklyn subway station.
"I know that people like the roughness" of New York subways, said David, "but it's a horrible way to die, and it can be prevented."
About 140 people are hit by New York City subways per year, in situations such as accidental knocks and willful leaps. Fifty-five people died last year and 47 in 2011, according to the MTA.
The numbers are small compared to the 1.6 billion subway rides taken each year, and officials say a substantial proportion is suicides.
But two men were killed last month in a scenario out of an urban nightmare -- each propelled into the path of an onrushing train by a mumbling stranger, in separate incidents. Then a stumbling woman fell onto the tracks and died when a train hit her early on New Year's Day.
Subway systems from Shanghai to Dubai to Paris have installed safety doors over the last three decades.
Sometimes called platform screen doors or edge doors, the devices are generally transparent walls or barriers that run the length of a train platform, with doors or gates that align with the train's doors.
They've been installed in more than 270 Tokyo stations and 530 throughout Japan since the early 1990s, according to the national Transport Ministry. Work continues on a 55-billion-yen, or $640 million, effort to install the doors throughout 29 stations that circle the city center.
Still, many stations don't have them, and more than 600 people a year fling themselves to their deaths on the tracks.
A 1990s London subway line extension included safety doors at eight new stations, for climate-control and safety reasons. London's transit authority says the cost of engineering the barriers into the rest of its 260 subway stations "would be quite prohibitive."
Some American transit agencies have eyed platform doors over the years, but the expense has led many to focus instead on simpler measures such as safety announcements, said Martin Schroeder, the chief engineer for the American Public Transportation Association, an advocacy group.
Still, there is some interest. The Federal Transit Administration is spending $275,000 to study platform doors and other ideas for minimizing passenger injuries. And some U.S. airport shuttle rails already have the barriers.