The May Amtrak derailment outside of Philadelphia put the sorry state of passenger rail in the United States briefly back on the public agenda, leaving many people wondering not just about the specifics of the crash but about the more general issue — how is it that a rich and powerful country that was a pioneer in railroad adoption in the 19th century has such terrible trains?

The United States is a big country, with lots of trains in it. So you can really think of this big generic question as composed of three separate questions with separate answers. One question, of urgent interest to media and political elites in New York and Washington, is why Northeast Corridor passenger rail service is so much slower than the first-rate systems found in France, Spain, China, and Japan. The second question, which will have bedeviled anyone who's ever been a tourist in Europe, is why passenger rail outside of the Northeast Corridor is so unimaginably awful. Last but by no means least, there's the question of why the richest and most powerful empire the world has ever known can't build itself a first-rate national, truly high-speed rail network along Chinese lines.

These questions are often lumped together under the hazy notion that American trains are bad.

1) Why doesn't the Northeast have a true high-speed rail?

Amtrak's Northeast Corridor — which runs between Washington, DC, and Boston with notable intermediary stops in Baltimore, Wilmington, Philadelphia, New Haven, and Providence — is by no means a world-class passenger rail system. But it's at least okay. And it's popular enough as a means of intercity transportation that its name even stands in as a name for the region.

It makes sense for this region to be a passenger rail standout. Compared with Japan or most European countries, the United States is a low-density and not-very-urbanized country. But the metropolitan areas connected by Amtrak's Acela service are an exception. The corridor contains about as many people as Spain and is much more densely settled.

Even better, a straight line drawn from DC to Boston essentially passes right through Philadelphia and New York, making the area ideal for train service.

Given the favorable geography, it is both understandable that this area features America's best intercity rail and baffling that the best we can do falls so far short of the best available in the world.

To a substantial extent, the Northeast turns out to be a victim of its own success. This portion of the country is so well-suited to passenger rail that its rail network was built out in the mid-19th century. Patterns of development were shaped by the route of the railroads, which, unfortunately, are not built to the standard of 21st-century speeds. Yet because the area is already rich and densely settled it would be extremely costly to build out a whole new set of straighter, faster tracks.

Amtrak has an official proposal to upgrade the region for true high-speed rail, but the price tag is $117 billion and many observers fear cost overruns.

A more realistic idea would be for the Northeast Corridor to undertake a series of cheaper incremental improvements that, in total, could drastically improve speeds. Transit blogger Alon Levy outlined such a plan several years ago, including a mix of construction projections, operational changes, and new equipment. There are, naturally, impediments of various kinds to implementing these ideas. But the big issue is structural.

The way Amtrak is currently set up, there's no real incentive to undertake incremental improvements. The Northeast Corridor already generates an operating profit, which simply defrays losses elsewhere in the system. Making it run better doesn't generate any wins for the people who would have to do the work, and would plausibly just lead Congress to reduce subsidies. If the NEC were spun off as an independent entity — perhaps even a private company — then it could internalize the gains from improved service and seek private financing to make cost-effective investments.

2) Why are trains outside of the Northeast so unbelievably awful?

A fundamentally different question is why — with limited exceptions —passenger rail off the Northeast Corridor is so incredibly awful. It's not just that these services aren't the best in the world and don't deploy the most cutting-edge technology available. They are often truly abysmal, with travel times worse than what was possible 100 years ago.

For example, if you want to go from Washington, DC, to Pittsburgh 250 miles away, you are looking at either a four-hour car trip or a seven-hour-and-43-minute Amtrak ride. What's more, there's only one train per day that makes the trip, departing DC a bit after four and arriving in Pittsburgh a bit before midnight. Of course, with the train so slow that there's no practical case for using it, there's little point in scheduling more trips. Megabus offers a slightly faster journey with two trips per day and charges $10 to $15, while Amtrak's fares start at $50.

This is not, it turns out, primarily a question of either incompetence or underinvestment. Instead the issue is that the dismal failure of US passenger rail is in large part the flip side of the success of US freight rail. America's railroads ship a dramatically larger share of total goods than their European peers. And this is no coincidence. Outside of the Northeast Corridor, the railroad infrastructure is generally owned by freight companies — Amtrak is just piggybacking on the spare capacity.

That means the technology isn't optimized for passenger rail needs. But it also means passenger train scheduling needs to take a back seat to freight priorities.

3) Why don't we build a state-of-the-art high-speed rail network?

But this is all legacy rail. If you look at the fastest, best passenger train systems in the world — the ones in places like China, France, Japan, and Spain — they're not doing a good job of managing 19th-century infrastructure, they're building brand new state-of-the-art high-speed rail infrastructure. To many people the question is: why not us?

And the answer is that it would be very expensive and it's almost nobody's first choice about what to do with the money.

How expensive? The truth is, nobody knows. At its most ambitious, the Obama administration asked for $53 billion on top of the $11 billion in stimulus funding for passenger rail. But that's clearly not enough for any kind of national system. Amtrak, you'll recall, said it needs more than $100 billion to do true high-speed rail in the Northeast Corridor alone. The rail skeptics at the Cato Institute extrapolated from what California is spending on its high-speed rail project to conclude that a full system price tag could be around $1 trillion.

Since California has above-average construction costs, that's almost certainly too high. It's also not the case that we couldn't afford to drop hundreds of billions on new passenger rail infrastructure over the next two or three decades. That's about what the United States spent invading Iraq, and a high-speed rail network would be more useful than that.

The real problem is that even among the set of people who are interested in the idea of a multibillion-dollar transportation infrastructure investment, intercity passenger rail just isn't that compelling a priority. Even if you just restrict your attention to train lovers, it's hard to make the case for intercity over regular urban transit. Amtrak proudly brags about the 11.4 million passengers who plowed the rails between DC and Boston in 2013. But America's 10th most popular rail transit system — Miami's not particularly useful or extensive two-line Metrorail system — hosted over 20 million trips last year. Cleveland's single 19-mile heavy rail route carried more passengers than Amtrak's second most popular line. Under the circumstances, new urban transit seems much more likely to be heavily used than new intercity trains.

What is true is that faster trains could offer a green alternative to plane flights between close-together cities — Seattle and Portland, or Dallas and Houston, for example.

But even though plane flights are very polluting, they only add up to a modest 2.2 percent or so of total American carbon emissions. And of course for many journeys — Phoenix to Boston or Houston to Seattle — trains aren't a very plausible substitute anyway. So from an environmental perspective, too, massive rail expansion just doesn't look like a particularly compelling priority.

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