I believe that Ecology, Economy, and Efficiency would all prosper greatly from inexpensive Electricity. Heating, Transportation, nearly everything can done better with Electricty IF it is cheap.
There are many innovative technolgies that could be employed to generate huge amounts of electricity, some space-based. And of course if Fusion ever becomes a reality all other forms of generating Electricty will become historical curousites. Even so our descendents will look back and wonder why we did not try harder to advance beyond Fossil Fuels more quickly.
But since this Discussion is supposed to focus on Mass Transit, I'll return there.
The distinction between Mass Transit and Private Transit will blur in the next few decades. What will make this possible will be changing the Driver. As computers advance, machines will be driving cars. Soon Manual Driving will actually become illegal. DWI, Traffic Lights, Road Rage, all gone. Each computer-controlled vehicle will do a delicate dance with thousands (or millions) of vehicles in the area, all moving in an efficient pattern to bring each passenger from Point A to Point B. As accidents will be rare, the vehicles will become lighter, with few safety restraints. These deletions from current practice will decrease the costs of these vehicles. And by controlling each Automat centrally, our entire Traffic System becomes far denser. We will be moving far more people, far more quickly, all on the same highways.
Tightly packed "Hives" will be built, holding thousands of vehicles packed door-to-door, bumper-to-bumper. Each morning these Automats will begin to stream out to pick up passengers. The assignments will be dynamic - the Server managing these Automats predicts how many vehicles need to be where and when. When the passenger is ready to leave, they'll make a request via Cell Phone (or what passes for one by then). An Automat passing nearby will be assigned and will arrive in seconds. Homes and Apartments will not require many Parking Spaces. Streets may require none.
There will be Budget Automats, Luxury Automats, Party Automats, Business Automats, Shopping Automats (for the big trip to Costco). You will rent them according to your immediate needs and preferences. And although you may take a Budget Automat TO Costco, you'll hail a Large Automat to bring you and all your stuff BACK.
It is obvious that travel by City Busses and Subways can never compare to the efficiencies of the Automat. These bumbling behemoths will disappear - and good riddance. Is there a Commuter alive that would not relish the privacy and humanity of being in their own space while headed to work ? Ok, for those who love to banter Social Automats will be employed, that hold 4, 6, or more, and these folks can yak it up to their heart's content as they travel.
Automats: our inevitable solution.