Scientific American has an explainer that breaks down how BART officials killed the transit system's base stations, disabling various cellphone services in an attempt to thwart planned protests. For about three hours on one day, transit riders were blocked from calling 911, making voice calls, accessing the internet or using any mobile data services.
The government-run public train authority did not, as some mis-reported, "shut off cell towers."
BART's wireless service is provided by a company called WiFi Rail. According to its web site, the WiFi Rail network utilizes a backbone of fiber-optic cables that run throughout the underground transit system. These connect a network of wireless access points, routers and switches. Because BART privately owns and operates this underground network, BART officials have the power to switch it off.