The plan to transform the city’s main post office into a grand rail gateway is back on track — because Amtrak is now on board, officials announced yesterday.

After six months of negotiations, the railroad has agreed to take part in the project, in which its ticketing, boarding and waiting areas would move from Penn Station to the James A. Farley Post Office building across the street.

Amtrak backed out of the plan in 2004, saying it would not pay rent or transfer its ticketing and waiting areas to the Farley building.

But under the new agreement, the railroad will get revenues from the stores and shops in the building and tracks will be built to accommodate the railroad’s high-speed trains, officials said.

Amtrak has stimulus money and will pour some of that into the new station — to be named for the late Sen. Daniel Patrick Moynihan — estimated to cost $1.1 billion to $1.5 billion.

The agreement was announced at a press conference by Gov. Paterson and Sen. Chuck Schumer.