Monorails & switches.
"The Simpsons" is a *cartoon*, not a fucking documentary.
Yeah, it's so effing hard to do monorails right and have proper, reliable switches on a technology that has been used FOR FORTY F*CKING YEARS IN ASIA.
Japan has seven monorail systems alone, most of which are proper metro lines with junctions and branches, just like any other metro, with similar capacities and frequencies. Those switches / points *have* to be reliable, so yes, it's been done. Repeatedly. And that's just Japan. Check out this list—http://monorails.org/tMspages/Asia.html—for Asia. (And before you ask, no, I'm not a member of that site. I just don't dismiss *any* technology.)
@Mike Richards: please re-read the above again, just in case you missed it. You are wrong. Demonstrably so. The traditional rail industry has its own agendas and even its fanbois, just like every other industry.
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Personal Rapid Transit is arguably a much better fit for medieval cities like London. (Imagine doing this—http://en.wikipedia.org/wiki/File:Edinburgh_Tram_Works_8th_August_2009.JPG—down Oxford Street, Peckham High Street, or Charing Cross Road.) A fully suspended transport system, be it monorail, PRT, or whatever, would keep the new network above the road, leaving it free for traffic. (Unlike cities like Rome and Paris, which are blessed with wide, arterial roads, London has rather fewer diversionary route options.)
While the use of maglev might seem over the top, it's not as mad as it sounds: you pay a bit more for the energy, but you save on maintenance as there's very little friction. You also get better acceleration and deceleration. The pods themselves are also much quieter and lighter as they don't have to carry a motor with moving parts around: the maglev track *is* the motor.
(That said, there's much to be said for the "Monometro" concept currently being trialled in the Middle East. This would be much closer to a light rail system and might be a better fit for London's tourist areas.)
Finally: artists impressions, incidentally, are just that: impressions created by *artists*, not engineers! That there are elements missing is hardly surprising.
When the London Borough of Lewisham held an exhibition a few years ago for their redevelopment proposals, I noticed that the two rivers through the town centre were shown about six feet higher than their actual level, suggesting that these would have nice, shallow, photogenically gentle slopes forming grassy riverbanks. Their large model and glossy images showed people sunbathing, casually walking alongside the Ravensbourne and Quaggy rivers, etc. In reality, they'd be sliding down a very steep bank into a river at the bottom of a bloody great ditch.