It has occupied a special place in New York City’s imagination for nearly 100 years: an unfinished dream, a punch line for delays, a construction nightmare. Some New Yorkers wondered if they would ever live to see the day when it came to life.

After decades of failed efforts, the first segment of the Second Avenue subway line is scheduled to open in December, with three new stations on the Upper East Side of Manhattan.

The chairman of the Metropolitan Transportation Authority, Thomas F. Prendergast, said in an interview last week that the line was set to open by the end of the year. It was important to show riders that “we live up to our promises,” Mr. Prendergast said. “We’re now within striking distance of having it done.”

Striking distance is one thing; up and running is another. There is plenty of work left to do, and workers are racing to make sure the line is ready on time. The new stations are nearly complete, and crews are operating trains on the tracks. But hurdles remain, including testing the fire alarm system as well as the escalators and elevators at the 72nd Street station.