1. Trains, because peak oil will kill planes and cars.
2. Bicycles, because peak oil will kill busses and cars.
It will take far more than 15 years just to build a new train line from LA to San Francisco, and those are two dense cities. And even people in New York prefer Vespas to bicycles. Of course, all that is irrelevant because the cities won't survive the first year of starvation once farmers try to pedal their crops a hundred miles to the nearest train station, which is actually feasible once their crops are limited to what they can sow and harvest by hand.