EVERETT — The CEO of Sound Transit assured the Everett City Council on Wednesday that the light rail extension that’s due to arrive in the city in 2036 won’t be held up by other projects in the system.

Last week the Sound Transit Board voted to study two tunnel alignments for future routes in Seattle, which, if ultimately approved, could delay and add $1 billion to the cost of those projects. Elected leaders in Snohomish County worried that move could impact other extensions.

The concern isn’t just over funding but also over the schedule of the Everett Link Extension, Mayor Cassie Franklin said during this week’s council meeting.

“As these options are explored, what is our safety net to ensure that whatever … ends up being the preferred option that is implemented in Ballard-West Seattle, it doesn’t interfere with the timeline to get to Everett?” Franklin asked.

Sound Transit cannot jeopardize one project for the benefit of another, said Peter Rogoff, the agency’s CEO.

“We as a matter of law cannot augment a project if we knowingly know it would endanger the delivery date or the funding of another project,” Rogoff said.

Councilmembers also questioned Rogoff about where third-party funding would come from to fill the gap in the budget if costs increase for the Seattle alignments. The move by the Sound Transit Board last week required outside money to be used if the scope of the project goes beyond what voters approved in the ST3 package in 2016.

Additional funds could come from the city of Seattle, the Port of Seattle, or by collecting extra tax revenue from appreciating property values and increased business activity near the route’s stations, Rogoff said.

This funding would not come from federal or state sources, Councilmember Paul Roberts stressed.

Roberts also sits on the Sound Transit Board.

The final decision on the West Seattle and Ballard alignments will be made in 2020. Proponents want the tunnels to minimize impacts to the main business area in West Seattle and the maritime industry near Ballard.

Sound Transit representatives also gave an overview of the progress of the 16.3-mile Everett Link Extension. The route, with six stations between Lynnwood and Everett, is expected to have between 37,000 and 45,000 riders a day.

“That will be our biggest project of all,” Rogoff said.

Early work has already begun, including engaging with the county and cities. This will allow the transit agency to hit the ground running in 2020 when planning officially starts, Rogoff said.

“Everything we know from all of our extensions to date, is that the earlier you start this work the greater certainty you bring to actually completing it on time,” he said.

Lizz Giordano: 425-374-4165; egiordano@heraldnet.com; Twitter: @lizzgior.