OK. Earlier political wonks, who go into politics young, study only political science (junk science?), and evade most other serious education, tested the idea of double decking and failed. So what?
Government’s job is to serve us. Not rule “The Masses” for our own good. And we love our cars for good reasons. 
Having lived in Boston w/o a car for 4 yrs when young, poor, and dumb here’s a few:
Big Box transport is a perfect Flu incubator (also pandemic accelerator). Never sicker. Terrorists prefer victims pre-packaged in Big Box transport (e.g. 200 dead/2000 hurt in Spain train). One car at a time lacks impact. One Big Box breakdown -Line and all its branches lost. Intelligent cars muddle around it.
It’s damn inconvenient, ‘cept maybe for promotors who see us clearing roads for them
LAND in Hawaii is priceless. Double decking doubles it. Rail consumes it.
Decking adds a covered lane for bikes and other cool wheeled things. We can build roads w/o building fortunes for CA Consultants
&more