Why there is not network coverage in San Francisco MUNI underground

I did not found answer on this , but I am fed up with long commuting underground without coverage.
BART underground have good signal.

I tried to contact support at http://www.verizonwireless.com/b2c/contact/index.jsp
Please send them request to raise this problem again an again: https://wbillpay.verizonwireless.com/vzw/secure/contactus/email.action

My question at forum:
https://community.verizonwireless.com/thread/787208