The $12.3 billion Gateway rail tunnel linking New York City and New Jersey has reached a major preconstruction milestone with the completion of geotechnical studies necessary for the engineering phase.

The analysis of rock and silt from 75 earth samples on both sides of the Hudson River marks the latest in a series of swift leaps toward a potential 2023 start date. The project had been delayed years by former President Donald Trump, who had argued that costs should be covered solely by the states, not U.S. taxpayers.

The samples, from depths of 48 feet to 505 feet (14.6 meters to 154 meters), will guide design, according to the Gateway Development Commission, the project’s overseer. Some areas of particular interest to the researchers were on Manhattan’s West Side, parts of which were underwater before landfill was added many years ago.