This trail will never reopen. Instead, the agency will construct a new trail under the light rail tracks a block away at NE 115th St.
The official detour is fairly out of the way and uses busy NE Northgate Way for a block. There are no bike lanes on Northgate, and the detour map suggests using the sidewalk. However, I suspect most people biking and walking will find their own unofficial ways through, likely using shorter routes through nearby apartment building parking lots.
As early as September 21st, Sound Transit is closing the Northeast 116th Street bicyclist/pedestrian path to construct the Lynnwood Link Extension light rail guideway. The NE 116th Street trail will be closed permanently. The Lynnwood Link rail-track alignment in this area changes from aerial (supported by columns) to at-grade (supported by retaining walls), similar to guideway being constructed just south of Northgate. Sound Transit will be constructing a retaining wall and track guideway for the Lynnwood Link light rail through the existing NE 116th Street trail. Sound Transit plans include a new pedestrian/bicyclist trail at NE 115th Street crossing under the new light rail aerial guideway.