X is for Streetcar

To direct traffic on San Francisco's streetcar network, the Muni transit system ("Muni" is short for San Francisco Municipal Railway) maintain signals in many locations. Many of those signals display X's to clearly indicate that these aren't traffic signals for cars. This signal is at West Portal in San Francisco, where the L-Taraval line joins the K-Ingleside and the M-Balboa Park to form a common route into the western entrance of the Muni Metro tunnel.

This photo was taken through the window of a streetcar in April 2012.