Redwood timbers being ripped out of the old tunnel.

Within a few weeks, there should literally be a light at the end of the Cal Park tunnel for cyclists commuting from San Rafael to the Larkspur ferry. Engineers are mucking out dirt and boulders on the collapsed south end of the 1,100-foot tunnel, making room for an 11.5-foot wide bike path and tracks for the SMART commuter rail trains.

Dave Bernardi, manager of the $25 million project, and Erin Hohenshelt of the Jacobs Associates engineering firm, gave a detailed update on the project to the Marin County Bike Coalition last night, predicting the 85-year-old tunnel will be open to cyclists by February 2010 “at the latest.”

The project includes a 1.2-mile Class 1 bike path and a self-contained “tunnel within a tunnel” replete with video cameras, lighting, ventilation, emergency phones and even fire hydrants, just in case. The bike passage will have its own walls and concrete ceiling, separating it from the 45 mph diesel trains a few feet away. The trains are expected to start rumbling through in 2012.

Construction crews have been busy removing the original redwood arches inside the tunnel and replacing them with steel braces. All went smoothly until the crews reached the south end, where they encountered loose dirt running through sinkholes and boulders the size of small cars.

“We have a lot of loose ground and cavities and we don’t see an end to it,” said Hohenshelt. But she cautiously predicted the south end would be opened up by May.

South side of the tunnel in Larskspur.

Hohenshelt, who commutes to the job site from San Francisco by bike and ferry said she often hears from cyclists who hope to be the first ones through. “But I can tell you who’s going to be the first one through,” she said with a grin.

The tunnel is at the south end of the planned 70-mile SMART train route from the Larkspur Ferry to Cloverdale. SMART was approved last year by voters in Marin and Sonoma counties. A quarter-cent sales tax to help pay for SMART took effect in both counties Wednesday.

The bike coalition has been working to reopen the tunnel since 1998, when its members thought the work might cost about $3 million. The project is supported by eight agencies: the County of Marin, Caltrans, the Transportation Authority of Marin, the Bay Area Air Quality Management District, the Metropolitan Transportation Commission, SMART, the Bay Area Toll Authority and the U.S. Department of Transportation.

MCBC is also leading an effort to reopen a second railway tunnel to connect Mill Valley with Corte Madera. That project, the Alto Tunnel, is one of three alternate routes being considered by the county. The other two – the Horse Hill bike path and Camino Alto – already exist but have been widely criticized by local cyclists as too steep, unsafe for children and unfriendly to pedestrians. However, the Alto tunnel faces opposition from local homeowners, who don’t want a steady stream of cyclists riding through their now-isolated neighborhoods.