It is quiet in Co-op City, the nation’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 on Friday afternoon. So calm that fewer than a dozen people boarded the express bus, the fastest link to Midtown Manhattan, for its hourlong 2 p.m. trip.

This middle-income development, with 35 high-rises and clusters of townhouses, is isolated from the rest of the northeast Bronx by two expressways and the Hutchinson River. The nearest subway stop is a local bus ride or a 20-minute walk away. With its own shopping center and schools, it is self-contained, so much so that some residents say it has been years since they have been to Manhattan.

A plan is afoot that might shift all that. Gov. Andrew M. Cuomo, in his State of the State address last week, revived a transportation proposal that has long been discussed: the creation of a new Metro-North rail spur that would link the New Haven line with Pennsylvania Station in Manhattan, with stops in Co-op City and three other eastern 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, Conn., in 31 minutes, providing a new link for Bronx workers to employers to the north and the south. It could also start a transformation in Co-op City, which remains, residents said, a bit of a real estate secret: It is a safe, leafy place where a three-bedroom co-op costs $27,000 to buy and less than $1,500 a month in maintenance fees.