The Regional Transportation District may issue a notice of default to the two contractors working on the N Line commuter rail extension to Thornton after blowing past its scheduled 2018 opening date.

“The fact that this project is still not complete is completely unacceptable and frustrating to not only us at RTD, but to our stakeholders and the public we serve,” said Angie Rivera-Malpiede, chair of the RTD board of directors.

A notice of default gives a contractor the opportunity to remedy a problem and explain why it is or is not in breach of contract. The next step would be to end the contract. RTD stated in a press release that the contractors indicated they are still committed to finishing the N Line as soon as possible.

And yet, the agency admitted that the line had not yet reached “substantial completion”.

The two companies involved, Balfour Beatty Rail Infrastructure and Graham Contracting, are in a joint venture called Regional Rail Partners. They issued a statement explaining that “the North Metro Rail Line project has experienced delays due to circumstances outside of the control of Regional Rail Partners. We remain committed to cooperating with our customer, the Regional Transportation District, and to the completion of the North Metro Rail Line project to bring enhanced services to the greater Denver community.”

The N Line broke ground in 2014 and is part of the 2004 tax increase to fund FasTracks, an expansion of the rail network in metro Denver. The N Line is 13 miles long and the terminus for this phase of the project will be at Eastlake-124th Station in Thornton. The travel time between Union Station and Thornton will be just under 30 minutes, with rush-hour headways of 20 minutes between trains.

RTD plans to expand the line by 5.5 miles to northern Adams County when funding becomes available.

Editor’s note: This story has been updated with a statement from Regional Rail Partners.