North Americans are in love with trains that go in highway medians. A large fraction of urban rail construction since World War Two, both light rail and full metro, has used highway medians as cheap at-grade rights-of-way to extend train service, often deep into the suburbs. Some proposed longer-range lines are supposed to go in medians as well: Florida had reserved space in the I-4 median for Orlando-Tampa high-speed rail, and Xpress West planned to go from Las Vegas to the outskirts of the Los Angeles area in the I-15 median. The Texas Central Railway, a private group backed by JR Central planning high-speed rail between Dallas and Houston, is considering several alignments, but markets the route as following I-45 (no mention of median) in some public discussions. In nearly all cases, both urban and intercity, it borders on incompetent to design rail lines in highway medians; intercity lines frequently follow highways on one side, but even that tends to be overrated in American discussions in my experience.
For urban rail, the reason to use highways is that, in most of North America, they’re everywhere, and they’re usually equipped with generous medians and shoulders, allowing relatively cheap placement of rail tracks. Of note, this is generally not the cheapest option: construction on extant (often disused) rail rights-of-way tends to be cheaper. However, in many cases, a rail right-of-way is unavailable, hosts heavy freight traffic, has been permanently turned into a trail, or has commuter trains without integration into the rest of the urban transit network. Examples include the Dan Ryan half of the Red Line and both halves of the Blue Line in Chicago, the Orange and Silver Lines in Washington, the outer ends of BART, the Spadina line in Toronto, and several light rail lines. Often they run on one side of the road, but more frequently they’re in the median, which was often reserved for it when the road was built (as in Chicago and Calgary).
The problem is that nobody wants to live, work, or hang out next to a busy grade-separated road. Living or working a kilometer or two away, with easy access by car, is great for the driver, but within close walking distance, there is just too much noise, pollution, and blight, and the pedestrian environment is unwelcoming. The transit-oriented development in Metrotown and Arlington could not have happened next to a freeway. Christof Spieler frames this as a decision of spending more money on routing trains near where people live versus staying on the easy rights-of-way. But this isn’t quite right: the Expo Line in Vancouver was assembled out of an interurban right-of-way and a city center tunnel, both out of service; the line’s high ridership comes from subsequent development next to Metrotown and other stations.
Other times, the routing comes from a deliberate decision to integrate the trains with cars, with large park-and-rides at the ends. This is common on newer light rail systems in the US (though not Canada, as Calgary prefers integration with connecting buses) and in the Washington and San Francisco suburbs. This makes things even worse, by extending the radius within which the environment is built for cars rather than for people, and by encouraging the same park-and-ride construction elsewhere, along abandoned railroads and greenfield routes, where the preexisting environment is not car-oriented.
I do not want to categorically say that cities should never build urban rail alongside highways. But I cannot think of a single example in which this was done right. Calgary is a marginal case: it did build light rail along highways, and had some success with transit-oriented development, but those highways are arterials rather than freeways, and this makes the pedestrian environment somewhat better.
The situation is somewhat different for suburban rail, but usually the scale of suburban rail is such that there’s not much new construction, only reappropriation of old lines. These lines are long and the environments low-density, making it hard justify the costs of new lines in most cases. Where new suburban rail is built, for examples the Grand Paris Express, and various airport connectors, it is typically in environments with such expected traffic density that the rules for urban rail apply, and we tend to see more underground construction or usage of extant rights-of-way.
The reasons favoring highway alignments intercity rail in the US are somewhat different. Tellingly, HSR in Europe is frequently twinned with motorways. It is not about integration with cars, since those alignments are rarely if ever meant to have major stops in their middle. Instead, it’s about picking a pre-impacted alignment, where there are fewer property takings and fewer NIMBYs. This logic is sound, but I often see Americans take it to extremes when discussing HSR.
The first problem is that roads are almost never as straight as HSR needs to be. The design standards I have seen after briefly Googling give the radius of a motorway capable of about 120 km/h as, at a minimum, 500-700 meters. With these curves, trains, too, are capable of achieving about 120 km/h – less at 500 meters without tilting, more at 700 meters with tilting. The most recent high-speed lines are built with a minimum curve radius of 7 km; about the absolute minimum that can be done, with design compromises and tilting trains, is 4 km. This implies that the trains have to deviate from the motorway alignment whenever it curves. In flat regions the road curves are much gentler than the minimum, but still too sharp for full-speed running. Both Florida HSR and Xpress West noted that the trains would have to slow down whenever the Interstate curved, because the need to run in the median would prevent them from curving gently enough to maintain full speed.
Of note, the European examples of HSR running in motorway alignments have it running alongside the roads, not in the medians. I invite the reader to spend a few minutes following French LGVs on Google Maps and seeing this. This is because there invariably have to be small deviations from the road, which in a rural area are trivial when one runs next to the road but require viaducts when one runs between the road’s two carriages.
There may also be an issue regarding reusing the Interstates. To transit supporters who view HSR as a replacement for freeways, this has an element of poetic justice, or just plain practical reuse of infrastructure they think is obsolete. I chanced upon this while looking up Interstate design standards, but I’ve seen similar proposals elsewhere, as well as dissimilar proposals making use of interstate terminology, as a reminder of past national greatness. It comes from the same place as proposals to reuse auto factories to produce rolling stock: there’s a romantic aspect in addition to or instead of an economic one.
But the most fundamental problem is that the contentious experiences of the freeway revolts and modern-day NIMBYism have soured Americans on any process that involves brazen takings. What I mean by brazen is that carving a new right-of-way, especially through a populated area, looks obvious on a map. In contrast, sticking to a preexisting right-of-way and incrementally widening it or straightening curves is less controversial, even when it involves eminent domain as well, and opposition remains much more local, based on the specific properties being taken, rather than stated in general principles. I am not completely sure why this is so; my suspicion is that widening and straightening are more easily justified as things that must be done, whereas a new right-of-way looks gratuitous.
In either way, Americans have convinced themselves that NIMBYs are a major obstacle to infrastructure construction. While zoning is a notoriously NIMBY-prone process, infrastructure often isn’t. In the English common law world, expropriations are if anything easier than in France, where farmers are especially powerful, or Japan, where rioters threatened to block the construction of Narita Airport. NIMBYs are good at getting their names out in the media, but when it comes to blocking construction, they are relatively powerless; California HSR is facing NIMBYs in the Central Valley, many of whom are conservative and politically opposed to the project regardless of local impact, but so far they have not managed to delay construction.
However, NIMBYs are a convenient bogeyman for public projects, as their motives are openly selfish. They give charismatic, authoritarian leaders the opportunity to portray their infrastructure projects as battles between the common good and backward-looking parochial interests. As I’ve noted multiple times before, New York’s livable streets community (which is similar politically to the set of HSR supporters in the US) tends to overblow the importance of NIMBYs to the point of seeing NIMBYs even when the concerns have nothing to do with NIMBYism: see, for example, the reaction to the opposition of two Harlem politicians to a plan to speed up only the whitest bus route through the neighborhood.