Last year, there were about 3.2 trillion passenger miles driven by urban drivers in cars in the US. My point about light rail is that we can barely afford it for just a few people, given that we spent $1.3 billion to build a rail line for just 17,000 daily round trip riders in Phoenix. If it were truly a sustainable technology, it could be applied to all commuters. But at a national average taxpayer subsidy per light rail passenger mile of about $2**, this means that to roll light rail out to every urban commuter would cost $6.4 trillion a year in government spending, almost half our annual GDP. If it required the subsidy rates we have in Phoenix per passenger-mile, such a system would cost over $12 trillion year. In fact, the numbers would likely be even higher in reality, because light rail in most cities is almost certainly built on the highest populated corridors with the most bang for the buck (though some of the diminishing returns would be offset by network effects).
Light rail only works today because we drain resources from millions of taxpayers to benefit just a few generally middle class commuters. This is not a model that will scale.
** This includes both service on the debt, which is payment for the original construction costs, as well as annual operating losses. This subsidy is required essentially forever. After 20-30 years when the original bonds are paid off, by that time systems generally have to be rebuilt in their entirety (as folks in places like Washington DC are learning). There are probably only 5-6 cities in this country that have the urban population densities to make rail systems come even in the ballpark of working financially, and places like Phoenix, Seattle, Houston, Portland and LA are NOT among them.