I believe you are thinking of the Key System (https://en.wikipedia.org/wiki/Key_System https://en.wikipedia.org/wiki/Key_System) which crossed the lower deck of

I believe you are thinking of the Key System (https://en.wikipedia.org/wiki/Key_System https://en.wikipedia.org/wiki/Key_System) which crossed the lower deck of the Bay Bridge. Amtrak never crossed it.