A "major milestone" in the long-delayed construction of a new Long Island Rail Road Terminal deep beneath Grand Central Terminal.

The MTA says it has started to lay track inside the 63rd Street tunnel that will eventually carry LIRR trains into Manhattan's East Side.

The massive terminal known as East Side Access is being built some 15 stories under Grand Central.

The $10-billion project has been plagued by repeated delays and is tentatively set to open in late 2023.

Officials on Monday said the project is now 72% completed and that 13-and-a-half miles of track will eventually be in place.

The MTA has said the new terminal will save Long Island and Queens LIRR riders up to 40 minutes a day in commuting time.