One other fact: Electric traction is less wasteful than internal combustion in providing for mobility.  If modern civilization is to continue to be mobile it is going to have to depend more on the underrated electron and not liquid fuel, be it petro- or bio- in origin.  

A multi-pronged approach would be best.  I'd shy away from putting all of our collective eggs in light rail or even rail in general.  In many cases it does make sense.  But other decisions transportation and land use-related must be made:

LAND USE
1) Are we going to try and preserve everything in existence or will we selectively abandon (and salvage for scrap) certain types of settlement or whole regions?
 -why try and extend service if the area has no longterm prospects.  
2)The opposite now, are there areas near existing infrastructure that ought to be more intensively developed to better utilze those resources?
3)Are there new forms of settlement going to form that concentrate residents in a single place, putting them close to agricultural resources, education, simple goods manufacture and basic service providers.  Think to various relocalization projects underway or discussed?  
4) What are we going to do about the "marginal places" where the addition of missing land uses (eg adding commercial and agricultural activities) make the place viable

TRANSPORTATION IMPROVEMENTS

1. sidewalks.  duh
   
2. bike paths/lanes and improved parking.  Most close tasks can be reasonablly be accomplished via biking. Definately short distance commuting.
   
3. bikes and bike technology - Improve the bike offerings to encorage: all-weather comfort, electric assist, expanded cargo capacity, safe children transportation, better delivery applications.  Many of these already exist (I use some already) but would benefit from lower costs and higher awareness.
   

4)Ridesharing/hitching network.  How many times have you gone somewhere only to find out someone else was going that way too?  A system that matches drivers to passengers would definately encourage more car pooling
5)Rail electrification. start with the mainlines and go from there. Encourage freight to be shipped this way.  container use helps this.  Double tracking and selective re-alignments in many areas will make freight travel faster and allow for the re-introduction of passenger rail.  Reactivate older branches and ROWs.

6. Urban rail extensions.  Use the appropriate rail tech.  Large cities should stick to heavy rail.  Can you believe one of the 2nd Avenue subway alternatives was light rail?  thats not appropriate.  Intermediate sized cities will benefit from light rail, the more grade seperated the better. If some small towns have legacy streetcar infrastructure in place and can put it to use, more power to them.  
   
7. Personal Rapid Transit- In certain circumstances, PRT would be ideal, mostly in large mixed use campus settings with no large peaks in travel demands.
   
8. Internet and telecommunications:  Even better, if you can accomplish something online or virtually, then you DONT have to travel somewhere.  How many trips has the internet already saved us?
   
9. Hybrid or battery trolley busses- Some places and locations it isnt practical to install rail.  In those cases run busses that can: (a) draw electricity from a wire in denser urban settings (b) run off battery power for longer stretches like up to 5-10 miles between wires and/or (c) run an internal combustion engine at an efficient setting to power a generator to keep charging the batteries.  This technology exists.  Plus if any transportation mode will benefit from fuel cells, the bus will be it.  A trolleybus-fuelcell combo may be the long term goal.  If we can only have to manufacture H2 from water via renewables, applying the fuel to municipal fleet vehicles could be probably quite manageable.  Again, use the overhead wires for urban areas, fuel cells for those areas in between
   
10. Electric Cars.  Screw the ICE engine.  Scratch the fuel cell.  Plug in hybrids (near term) and straight-up electric cars need to be the way of the future.  If we cant do that, at least mode choices 1-9 are still available.  I'd advocate a stronger future for electric cars IF we had...
    
11. Dual-Mode. -  Even better.  Imagine a network of elevated monorail-styled track network that you'd drive your electic vehicle onto and proceeding automatically, drawing current directly from the rail.  Dual mode combines the practicality of the electric car (short distance travel) and gives it long distance potential.  Now you could drive only up to 5-10 miles, enter the rail system, being guided automatically and then exiting and driving the remaining 5 or so miles to your destination.  Public transit applications would include creating bus runs that took advantage of rail locations to create numerous flexible running lines, dial a ride and so on.  A simple offline station (like for PRT) would allow totally automated PRT style use within the rail network.
    
12. Sail assisted shipping.  Maybe not going back to preindustrial transoceanic vessels but more modern sail arrays to reduce fuel usage.
    
13. Dirigibles.  If for some reason we can no longer fuel our airplanes, we could still take to the sky in airships.  Just need some hydrogen and fair weather to make this work.  Any takers?  Granted these things didnt have the best track record in the thirties but is there any reason why they wouldnt work with some more modern design elements.

If all else fails, walking works for most of us.