Somebody in Houston is going to have one hell of a smug look on his/her face when gasoline hits $3 a gallon and they are riding that train downtown to work. Public transportation in heavily populated urban centers is the intelligent alternative. I don't care what city it is.
There are several problems with that.
First, in order to ride a train downtown to work a train must go from the suburbs to downtown. Metrorail doesn't and Metrorail's planned expansions will not. Houston's suburbs are 20-30 miles outside of the city and Metrorail, both now and 20 years from now when Phase II is complete, will not service any area more than about 6 miles outside of downtown.
Second, Houston's urban center is not heavily populated, never has been, and likely never will be. It's a decentralized city with low population density. Nobody lives downtown save a few highrises and apartment complexes. It would also be an inefficient use of land to build more apartment complexes or highrises there as doing so would consume more lucrative real estate from commercial and hotel uses. Therefore your assumption about rail being an "intelligent alternative" does not hold.
Third, it does indeed matter what city it is. Houston is not New York and New York is not Houston. Take an example: Building a subway system in Houston, for example, would not work even if New York has it. Why? Because Houston's land is clay-based and prone to flooding. Take another example: building an oil refinery near downtown NYC won't work like it does for Houston because there isn't nearby oil pumping into NYC to be refined. You'd have to ship it over long distances from somewhere else.
On the off chance that you might be correct, let me point out that Houston scarcely fits that bill: it has a downtown, alright, but people commute there from all over the countryside, not from one central point you could put a train station.