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Look! Up in the sky! It's a train!
On flyover interchanges, population density, and the weird shape that might be the only hope for high-speed rail to take in America
It is easy to forget or simply overlook the fact that the United States has only a fraction of the population density of many of the countries we consider our peers and near-peers in the advanced industrialized world. Canada and Australia are both big and sparsely-populated, but Germany has more than six times as many people per square mile as the United States, the UK has more than seven, and Japan has more than nine.
■ The difference in density makes a big difference to many of our policy choices. Approaches to environmental protection, crime prevention, and housing are all influenced by density. But even more visible is the effect on how we choose to establish our transportation infrastructure. The beloved American open road would be a lot less open and would hold a lot less allure if there were 5 or 10 times more drivers per mile of roadway.
■ The accelerating adoption of autonomous and semi-autonomous vehicles promises to let us concentrate more vehicles on the roadway at a higher density and with much greater safety than has been possible up until the present. And there is good reason to believe that we could be approaching the advent of autonomous aircraft -- in effect, flying buses, if not quite flying cars -- that could also transport people without requiring live pilots to do the work.
■ Coupling these technologies with improved electrification could well mean that many of the traditional incentives to build other forms of high-density mass transportation may go by the wayside. But there remain people who are very enthusiastic about the idea of getting America to match its peer countries with the development of high-speed rail access, and they include the Secretary of Transportation.
■ If the United States were ever to have a high-speed rail network that were national and continental in scale (rather than localized or regionalized, like the Acela in the Northeast or a hypothetical Pacific Northwest network), then there is really only one way to see it coming about: That would be for a brand-new high-speed rail network to be overlaid directly on top of the existing Interstate highway system.
■ To achieve this would require considerable ingenuity. In most places, it would be wildly impractical to try to place rail lines either in the median between lanes of opposing traffic or along the shoulders -- at least, impossible at grade level. Which means that the only plausible scenario is for such a train system to be elevated above the roadways, perhaps by quite a lot.
■ This scenario could open the door to a potential strategic advantage that such a rail network might have over both road and air transport. It is possible to imagine a suspended monorail system with an enclosed rail configuration. The enclosure would provide intrinsic weather protection, making the trains useful for all-weather, high-speed transportation; a system that, if designed robustly, may not be subject to the same constraints as either aircraft or road vehicles.
■ The heights required to provide smooth service over the many bridges that cross the Interstate highway system might at first seem like a major obstacle. But in many places, motorists have already become familiar with flyover or stack interchanges, which have replaced the traditional cloverleaf design for many high-speed intersections.
■ As we have seen roadways get stacked two and three levels tall, we start to engage the possibility of imagining an elevated rail network reaching just as high -- but with far less concrete and without the risk of spinouts in icy conditions. Support pedestals could be installed in the medians of interstate highways, which conveniently have already been engineered not only for the necessities of matters like drainage, but also for accommodating relatively high-speed traffic.
■ The idea certainly would take some getting used to, and a massive amount of financial investment. Those those two factors alone make it relatively unlikely to ever come to fruition. But proponents of high-speed rail are a tenacious lot, and it's possible to see the right combination of advocacy mixed with political enthusiasm for infrastructure investment converging to make it happen. And if, for example, such an elevated rail system were able to, for instance, double the speeds achieved by vehicles on interstate highways, while performing safely during weather conditions that would ground aircraft or impede road traffic, then all-weather reliability may in fact be the killer application for high-speed rail.