Key System Mole

The Key System Mole was an interurban train and ferry pier on the San Francisco Bay. It served as an interchange point in the East Bay for Key System passengers traveling to and from San Francisco. It opened to passenger service in 1903 and was upgraded several times until 1933 when it was partially destroyed by a fire. Passenger service ended in 1939, and segments of the structure were partially reused in construction of the San Francisco–Oakland Bay Bridge.

Opening, popularity, and expansion
The troubled California & Nevada Railroad had begun construction of a ferry pier in Oakland, but its plans were never realized. Francis Marion "Borax" Smith purchased the railroad in order to gain access to its right of way and waterfront operations, as well as use the abandoned pier as a starting point for his own passenger mole.

Transbay service began on October 26, 1903. The original station building was designed by Walter J. Mathews and featured three tracks with one ferry slip. Switches were initially operated track-side until February 20, 1905 when a tower was installed.

As demand for the service greatly exceeded expectations, the terminal was enlarged several times beginning very shortly after opening. Six tracks were in use by July 1907 with two more being constructed. The hasty building methods employed during construction were also becoming apparent at this time. The original approach trestle was built with supports very far apart and potentially reused part of the original California & Nevada Railroad structure; this was replaced with landfill in 1908. By 1916, most of the trestle bridge had been replaced with causeway, except a 3800 ft section connecting to the ferry terminal. The rebuilt approach to the ferry terminal was designed to allow for construction of a loop at a future time. Three ferry slips were operating by the 1920s.

The Oakland, Antioch & Eastern Railway began running interurban cars from Sacramento to the mole starting in September 1913. This railway would go on to be consolidated into the Sacramento Northern Railway, offering services as far north as Chico and Oroville.

On December 4, 1924, one train collided with a stationary train at 40 mph, killing six people and injuring 45 more.

The Key System expanded their maintenance facility along the causeway in 1930 with the construction of the Bridge Yard building.

Destruction and rebuild before bridge service
In May 1933, an explosion and the ensuing fire destroyed the station building, 500 yd of pier, the ferry Peralta, and 14 cars. The cause of the fire remains a mystery. The Key System received a $1.1-million insurance settlement for the disaster ($ in adjusted for inflation). With bridge construction looming, the pier and station were rebuilt as cheaply as possible to maintain service. A temporary steel structure served as the new station building. The line and station were rebuilt slightly to the south to accommodate bridge construction.

Initial concepts to build a bridge between Oakland and San Francisco included a design to extend the Key System Mole to San Francisco. Final plans for the San Francisco–Oakland Bay Bridge included building the Oakland approach adjacent to the mole. After the fire, the California Toll Bridge Authority filed to condemn and acquire part of the causeway as well as the property of the former terminal. Transbay service was transferred to the bridge on January 15, 1939, though Key System cars and ferries continued to run to serve the Golden Gate International Exposition until the season's closing on October 29. (Ferry service from the East Bay was not resumed the following year, and was replaced with buses.)

Eventual fate
The Mole was almost entirely dismantled over the following decades. The steel structure which served as the station building after 1933 was sold and moved to Oakland, where it served as part of a steel company's processing plant into the 1980s. 600,000 board feet of timber was salvaged from the pier for reuse in construction of the Shipyard Railway. The Bridge Yard building was restored by the California Department of Transportation in the late 2010s and serves as an events space. Portions of the causeway exist as landfill approaching the Bay Bridge.

Design
The original wooden trestle extended 17000 ft into the Bay. After being rebuilt between 1906 and 1916, the trestle segment had been shortened to 3800 ft, the rest being built up with landfill to become a causeway.

The Mole's precarious length and ferry slips were the inspiration for the "Key System" moniker; W. F. Kelly, the railroad's first manager, believed their depiction on maps resembled the shaft and teeth of a key.