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DENVER — The RTD North Metro Rail Line will take longer to finish than expected, officials said Tuesday.

The first 13 miles of the N line commuter rail project are being re-evaluated by RTD and Regional Rail Partners, RTD said in a news release.

The first phase of the plan is an 18.5-mile electric commuter rail line that is supposed to be finished by the end of 2017 and opening in early 2018. But those plans have been scrapped because of construction challenges, RTD said.

The first 13 miles of the line are funded and are under construction from Union Station to the Eastlake 124th Avenue Station in Thornton.

The main contributing factor impacting the N line schedule is the completion and approval of the overall design drawings.

RTD has not given an exact date for completion.

“In any type of construction — housing, roadways, commercial buildings — there can be schedule impacts for various reasons,” N line project manager Ashland Vaughn said.

The N line will connect Denver and Adams County by serving Commerce City, Northglenn and Thornton as part of RTD’s FasTracks plan.

RTD said the delay is not related to current commuter rail issues. It said the A, B and G lines are separate projects.