The construction of the Second Avenue subway on the Upper East Side has been an epic odyssey, and after decades of false starts, the first phase finally rolled with the start of the new year.

The new subway line officially opened to the public at noon on Jan. 1, 2017.

The journey to launch the line goes back to the 1920s, when the route was first proposed by a state agency. Construction was derailed by the Great Depression, World War II, the city’s financial woes in the 1970s as well as the challenges of building in one of the densest neighborhoods in the country.

The state finally broke ground on the first phase of the line in April 2007, although construction work had been undertaken in previous decades in Chinatown and East Harlem.

MTA officials say the Second Avenue subway will allow residents and commuters easier access to mass transit on the city’s East Side.

The subway line, the first major expansion of the system in more than 50 years, will span 8.5 miles from 125th Street in Harlem to Hanover Square in lower Manhattan.

“There are many benefits. It will give the Upper East Side more travel options that they didn’t have,” said board member Andrew Albert, chairman of the Transit Riders Council.

Here’s what else you need to know about the Second Avenue subway.