I'm pretty sure being able to use your cell phone to dial 911 is a basic human right in America.
You can pick up any cellphone that has a battery in it, regardless of contract status, and call 911. That is of course unless you ride BART during a protest, a time when you are much more likely to be in need of emergency services.
Sure, given that you have reception. Is BART obligated to provide you reception?
If you say yes, how about the USFS? When I'm up in the mountains in land managed by the USFS, I cannot always place a 911 call. Can I sue the USFS for obstructing one of my most basic human rights, access to rapid sophisticated medical response?
Do the mountains have cell towers that are shut off when it is politically inconvenient? If it does, then yes, I would recommend legal action if you needed to contact emergency services and couldn't because they went out of their way to shut them off.
BART is fairly dangerous. It has it's own dedicated police force. Is it ok for movie theaters to jam cell phones? What happens when someone dies because they couldnt get thru to 911 during a protest in a major city? What about when they shut the Internet off during a riot in NY?