Report Reveals Alto Tunnel Options

MARIN COUNTY, CA – Marin County residents are getting a chance to learn about the options for the long-closed Alto Tunnel, a former railroad passage between Mill Valley and Corte Madera, including reconstructing it for non-motorized transportation, sealing it for good, or taking no further action.

More than three decades after it was closed, the 132-year-old tunnel is the subject of a new report and the findings will be presented at a community meeting at 6:30 p.m. Wednesday, Sept. 27, at the Corte Madera Community Center, 498 Tamalpais Drive. The tunnel's options and related cost estimates are outlined in detail in the report.

The purpose of the study was to supply updated information for further public discussion. The report concluded that it would cost about $46.8 million to reconstruct the tunnel as a multi-use pathway or $8.5 million to permanently seal and abandon it. Factoring for inflation, the cost options for the tunnel are close to those estimated in the 2010 Mill Valley/Corte Madera Corridor Study. With study results in hand, Marin County Department of Public Works is more informed about the methods required to reconstruct the tunnel, including a lower estimate of the construction cost contingency. A third option, as presented in the report, would be to leave the tunnel as it stands. No decisions have been made at this time about subsequent action, nor will any decision be determined at the Sept. 27 public informational meeting.

The study consisted of boring five holes total at two locations above the tunnel right-of-way – Underhill Road just west of Upperhill Road and Chapman Drive at Camino Alto – and lowering cameras and scanners into the tunnel to determine the interior’s condition. The process avoided significantly higher costs of cutting a portal into the tunnel to physically enter it and lessened the impacts to nearby homes and environment. The Alto Tunnel Investigative Study cost approximately $646,000 for the geotechnical investigation and real estate research.

As a result, the DPW knows where the 2,200-foot tunnel is intact, where it shows signs of deterioration, and where groundwater is present. The study findings, including photographs and scans of the tunnel’s interior, will be presented at the Sept. meeting.

If reconstructed, the Alto Tunnel would be an alternative and more direct route between central and southern Marin than what is currently available for non-motorized traffic. Bicyclists and pedestrians traveling between Corte Madera and Mill Valley have the option of the Camino Alto/Corte Madera Avenue roadway or the Highway 101 "Horse Hill" bike path. Both choices create challenges for travelers because of the steepness of those routes and the close presence of motorized vehicles. If rebuilt, the Alto Tunnel would provide a safer, more attractive and relatively flat non-motorized route between the two communities.

More information, including the full report, is available on the study's website.

– Press release and photo submitted by County of Marin