I'm a senior in Transportation Design right now and we get asked this a lot. Last semester i competed in the Michelin Challenge Design. My group won "Best Transportation System".
Our concept was made for Mexico city and we each designed a different car that would be able to drive on roads or convert to rail travel. Mine also converted to off-road to take advantage of the many
Parques Nacionales just out side of the city. Unfortunately the computer model didn't come out as I'd hoped but here is a sketch of the Voisin-inspired car.
Green sketch rear by
Tyler Linner, on Flickr
I personally was not on board totally with the rail idea because I thought despite my car's titanium wheels the extra weight would not be worth the reduced rolling resistance. My ideal future mobility concept would be cars that can sense lanes, speed limits and other cars well enough to drive themselves for long distances. This would allow them to use fuel-efficient driving practices and drive closer together to reduce congestion, not to mention give their occupants time to safely put on their makeup, read the newspaper or what have you. In the 90's a small stretch of freeway was paved with small magnetic markers that specially equipped Buicks could sense and navigate successfully. If we could do it entirely with optics there would be no added infrastructure cost.
DARPA has proven this to be possible through their robotic car challenges. I believe Google is running some driverless Street View camera cars as we speak. All we need to do is get this to a level that can be safely used across the board. The problem is even if it's 99.9% accurate, that one person that gets killed in an automated car will bring the whole thing down.
As far as propulsion I think a diesel generator giving juice to electric drive motors is the best solution for right now, but in the future with thin film battery cells we should be able to see pure EV's competing in the marketplace. In the meantime the Volt and Leaf should stay on the market to keep development going. They are going in the right direction in my opinion.