The Campbell-Almaden Railroad is based on a Southern Pacific branch line that ran between Campbell, CA and New Almaden, CA, and whose major industry was the New Almaden Quicksilver Mine.  In reality, the branch line fell out of use in, I believe, the mid '50's and the mine was closed for good in the late '70's.  The fictional Campbell-Almaden took up that branch line to service the mine and other customers in between during the '70's and beyond, and interchanges with the SP and WP into the Central Valley of California.