The SP Colma Branch (or San Bruno Branch), like the 4th Street Line in San Jose, is part of the original alignment of the Coast Line Route. It opened for service in 1863, and was downgraded to branch line service in 1907 after the 1906 earthquake prompted the building of the "Bayshore Cutoff," which is the present-day alignment of the line, which is now owned by Caltrain. The line also featured steep grades and required helpers during its time as a mainline.
The line was severed in 1942, and a few portions remained to served industrial customers. The line was abandoned piece by piece, as the 280 Freeway was built over portions of the line and later, BART (Bay Area Rapid Transit). It was likely abandoned sometime in the last few decades.
Thanks to Paul Carr for contributing information about this route.