NEW YORK — Three contractors have been picked to work on the expansion and transformation of Manhattan’s Penn Station, New York Gov. Andrew Cuomo announced Tuesday, a key step in the construction of what he called a “world class, 21st-century transportation hub.”

The three companies selected for the work are Skanska AB, Vornado Realty LP and Related Companies.

The project involves redeveloping the historic Farley Post Office into the 255,000-square-foot Moynihan Train Hall to serve passengers on Amtrak and Long Island Rail Road trains. The facility will also have substantial office and retail space.

Plans are also underway to overhaul the existing Penn Station. In January, the state projected the combined project will cost $3 billion. Private developers are expected to pay a sizeable amount of the total in exchange for commercial rights.

Penn Station now serves 650,000 passengers per day — three times the number it was designed for — making it the busiest train station in North America. Cuomo said the existing facility “is dirty, it is dingy, it is dark … it is decrepit and it’s an affront to riders.”

“The new train hall that we are building will be magnificent,” Cuomo said. “New York will not have seen anything like it in decades and decades.”

Work is set to begin in January, with the hall to open in 2020.