The Dart underground project, shelved a decade ago following the economic crash, is being resurrected by the National Transport Authority (NTA), documents reveal.

The €4.5 billion underground line to link Heuston Station to the Dart line by tunnelling under the city was one of the “big three” transport projects planned during the boom, along with the Luas Cross City and the Metro.

While the new Luas opened in December 2017, and a planning application for the Metro is due to be submitted to An Bord Pleanála shortly, the underground – by far the most expensive of the three projects at the time – was shelved in 2011.

However, according to The Irish Times, the NTA is again pursuing the underground scheme, which will be one of the largest transport infrastructure schemes undertaken in the city in decades.

The NTA has engaged consultants Jacobs Engineering to devise route options for the line.

The new line, branded Dart+Tunnel, would see a tunnel portal starting at the Docklands and ending just west of Heuston Station, south of the Chapelizod bypass and close to the National War Memorial Gardens.

A spokesman for the NTA told The Irish Times the Jacobs study would be published in the middle of this year as part of a public consultation process on the review of the transport strategy for the greater Dublin area.

The NTA last August announced plans for the extension of Dart services from Dublin to Drogheda, Celbridge and Maynooth – with the electrification of services on the three commuter lines; purchase of new Dart trains; and upgrade of the existing southern Dart line to Greystones. However, the underground was not included in these plans.