This line was once part of the Southern Pacific's main line through San Jose, CA, the Coast Line. While the mainline was relocated to where it is today, this line remained as a branch of the mainline; it was called the 4th Street Line, since part of it ran in the pavement of its namesake street.
Today, most of the line is abandoned, except for a small central portion of it, used to connect to the Luther Industrial Spur, still in use by Union Pacific.
See also the The Colma Branch.
Thanks to Paul Carr for contributing information about this route.