The fire inside the tunnel east of Cloverdale was reported around 6 p.m. Wednesday, Cal Fire communications operator Jeannine Davila said.
It was structurally unsafe for firefighters to enter the tunnel east of U.S. Highway 101 and it was decided to let the blaze burn itself out, Cal Fire said Thursday.
Davila said firefighters flooded the tunnel with water instead.
The blaze emitted dense black smoke and Cal Fire is still receiving phone calls about it, Davila said.
The tunnel is located west of River Road, south of the Geysers Road on-ramp and north of state Highway 128, Cloverdale Fire Department engineer Peter Avansino said.
The cause of the fire has not been determined, Avansino said.
Northwestern Pacific Railroad Co. is expected to resume freight train service between Sonoma and Napa counties this month for the first time in 10 years.