Rail trestle, vegetation burns in grasslands near Dumbarton Point in Fremont

An out-of-service wooden railroad trestle just south of the Dumbarton Bridge west of Fremont was burning Sunday night, and appears related to a vegetation fire reported in the area at about the same time late Sunday afternoon, a Fremont Fire Department battalion chief said.

The fire was reported about 5:12 p.m., and burned about three-quarters of an acre near Dumbarton Point, said Battalion Chief Kyle Adams.

The fire then spread to the trestle, and as of 7:30 p.m. Sunday, about 100 yards of the old wooden structure was burning, including a stretch west into San Francisco Bay, Adams said.

Both the trestle and the grasslands proved hard to reach, Adams said, taking firefighters almost 30 minutes to reach it.

Firefighters needed to use four-wheel-drive rigs to reach the fire.

Thick plumes of black smoke were rising from the trestle Sunday night. Twenty-one Fremont firefighters, plus a crew from the Alameda County Fire Department, were fighting the fire.

Adams said the cause hasn't been determined, but that it was likely human-caused.

The old rail bridge, which connects Newark and East Palo Alto/Menlo Park, hasn't seen trains for decades.

But Facebook, whose main campus is near that rail line in Menlo Park, is negotiating with the San Mateo County Transit District to improve the Dumbarton corridor, including reviving the rail bridge, for operation by a commuter rail agency.