I don't think cars caused urban sprawl so much as they caused urban merger. The Tucson area (I always have to deal with home because that's where I know) used to be a dozen different little towns. Most of the area now known as Tucson was either a town in its own right or space between town (which might or might not be developed). But rapid transportation caused all those places to get gobbles up. In sheer land mass Tucson is a huge city (about 25 miles by 15 miles), but it grabbed onto the motife of the west (multi-story building are the exception here, unlike back east).
The Miami-Jupiter line sounds like an extreme example of the basic problem all mass transit has. Basically they can be considered under two words: stops and schedules. Unless you're one of the luckiest SOBs on the planet no form of mass transit goes from your front door to the front door of where you work. This isn't anybody's fault, it's a matter of logistics if the bus stoped at every single house it would never get anywhere. On top of that you have to coordinate the you schedule with theirs, often you have to make connections so you also need to coordinate their schedule with itself. That's the stuff cars provide that mass transit (at least as it is currently understood) can never beat.
Now, in the spirit of goodwill, cause deep down I am a nice guy just don't tell anybody, I will throw out an idea for mass transit that can beat the car. Now, there are some other logistical nightmares that might make this idea impossible, certainly it's more of an urban solution than a method of connecting urban areas but if cities were wired this way then the train would actually not suck. I put that proviso there because you might thinking I'm poking fun when I put forth the idea but I'm not, it's just an idea with serious technical limitations. Here goes: people movers. Anybody that's been in one of the large airports in this country has seen them, as a kid I always thought they were the coolest part about being at O'Hare. Endless belts, possibly with seats in one area, replacing the sidewalks of our nation would solve the majority of our transport problems. There are major technical problems as I said, but if you think about it they do all the things mass transit currently cannot and it's more efficient than cars. As you pointed out walking give you the same stop and start conveniences as a car, but as I said walking sucks. Sitting on a chair connected to a people mover gives the best of both worlds. Just not sure how they interact with motor vehicles (which we'll still need, if only delivery trucks to supply stores). And I'm not sure how the chair idea deals with the end of the belt. And clearly you won't be able to sit in one chair for the whole trip because you'll prboably switch belts to make turns at intersections. But the basic idea is there, and, unlike trains, it's actually a technology that's newer than the car and more likely to be able to replace the car.
Just a thought anyway. I've noticed from these threads that most of the pro-train folks focus on how to get from town to town. While most of the pro-car folks (myself included) focus on how to get around town. For anything to replace the car it will have to solve both equation at least as well as the car does (which, as you guys note, the car doesn't do a good job of solving the multitown travel equation, so it won't have to be real good at that). What you need is something that gets me from work to home via the mall and the grocery store about as well as the car, which trains will never be able to do, but probably something else can.
But strictly on an aesthetic basis, I still maintain that cars have damaged our cities. No one can convince me that my hometown in Indiana, for instance, really needed five new strip malls or that they have someone improved the quality of life of Eastsiders. As I see it, developers bought up land on the cheap, built stip malls on the cheap, and rented the strip malls out to national chains with economies of scale. The national chains lowered prices and benefited from massive advertising campaigns. The smaller mom and pop shops closer to downtown lose business, and ultimately go under. Eventually, everyone finds themselves driving an extra 15 minutes to get to WalMart or Sams or Outback Steakhouse, and more farmland gets swallowed up in concrete. Can people buy toilet paper and ground beef and prime rib a bit cheaper than before? Absolutely. But we all miss the way our town used to be, with the little family owned shops and the vibrant Main Street.
I'll concede that my argument is more aesthetic than economic--and therefore may not be much of an argument at all!
As for the slow leeching of talent and youth from our small towns, I blame that on corporatism run amok rather than the car.