It's quiet at Co-op City, the country's largest housing cooperative, even though some 50,000 people live there. So quiet that the sound of footsteps crunching on icy sidewalks was louder than the wail of distant police sirens Friday afternoon. So quiet that fewer than a dozen people boarded the express bus, the fastest route to Midtown Manhattan, for a one-hour ride at 2 p.m. This middle-income development, with 35 skyscrapers and clusters of townhouses, is isolated from the rest of the neighborhood. northeast of the Bronx by two highways and the Hutchinson River. The nearest metro station is a 20-minute walk away by local bus. With its own shopping center and schools, it's self-sustaining, so much so that some residents say it's been years since they've been to Manhattan. A plan is underway that could change all that. Gov. Andrew M. Cuomo, in his State of the State address last week, revived a long-discussed transportation proposal: the creation of a new Metro-North rail spur that would connect the New Haven to Pennsylvania Station in Manhattan, with stops at Co-op City and three other East Bronx communities along the way. If the spur is ever completed, Metro-North says commuters could travel from Co-op City to Penn Station in 27 minutes, and to Stamford, Connecticut, in 31 minutes, providing a new link between Bronx workers and employers from the north and south. It could also begin a transformation at Co-op City, which residents say remains a bit of a real estate secret: It's a safe, leafy place where a three-bedroom co-op costs $27,000 to buy and less than $1,500 per piece. month of maintenance costs. Tatiana Markaryan, for example, has lived since 1993 in a two-bedroom apartment on the 21st floor with a balcony. She and her family don't have to change buses. About 12,000 Co-op City residents are over 60, and some, like Ms. Savlowitz, have been there since the development opened 40 years ago. “For the first 30 years I lived here, the botanical gardens were like my backyard,” she said. Co-op City, managed by RiverBay Corporation, is a Mitchell-Lama development, a form of subsidized affordable housing for middle-income buyers. . Sales prices are fixed, based on $4,500 for each room in an apartment, and there are maximum and minimum income requirements for buyers. For a five-room apartment, for example, the income range for a family of four is between $35,000 and $116,000 per year. But while hundreds of people are on waiting lists, the wait can be as short as a year for some apartment sizes, which is shorter than at some other Mitchell-Lama developments, according to one site State web that estimates expectations.