The night before last, a Northeast Corridor Amtrak train derailed in Philadelphia, killing seven people. For some overviews of what happened, see Vox and Huffington Post. I am not going to talk directly about the accident here; it appears to be the same kind of derailment as on Metro-North a year and a half ago. Instead, I’m going to talk about the general issue of redundancy, which I saw people bring up in response to the train shutdowns that followed the crash. This is not the first time I hear about this; redundancy figures prominently into the list of benefits touted for new rail tunnels across the Hudson, allowing Amtrak to shut down the existing tunnels for repairs. Even before Amtrak proposed the Gateway project, transit activists talked about redundancy as a positive feature, for example Cap’n Transit. In this post, I am going to explain why, in public transportation and intercity rail, redundancy is in fact far less useful than other investments for the same amount of money.
First, let us list the various high-caliber rail networks of the world. In high-speed rail, the biggest networks are those of China, Japan, and France. None of them has redundancy, in the sense that there is more than one way to get between two cities on high-speed track. JR Central is building a second line from Tokyo to Osaka, but this is because the existing line is at capacity, running about 14 trains per hour into Tokyo at the peak; redundancy is a minor consideration. In regional rail, the busiest networks do have some redundancy, in the sense that if one line is shut down then people can take a parallel line, but this is because these networks are so busy that in most directions there’s enough demand to fill multiple lines. In Tokyo, which has the largest regional rail network, the parallel line is usually run by a competing company, so within each company’s network there’s little redundancy.
The reason for this non-redundant operation is simple: building new rail lines is expensive, while maintaining them adequately so that they don’t break down is cheap. Amtrak thinks that the Gateway tunnel will cost $16 billion. The program to repair the damage the preexisting tunnels suffered in Hurricane Sandy is $700 million, which assumes an accelerated construction schedule in which the tunnels will be shut down one track at a time, but conversely also includes work in the worse-damaged East River tunnels and not just the tunnels across the Hudson. This is a one-time repair after salt water intrusion, not annual ongoing maintenance. New Hudson tunnels are a necessary project for capacity reasons, but whatever benefit they have for redundancy is a fraction of their cost.
For high-speed rail, too, the costs of maintenance are far smaller than those of construction. The average maintenance costs of a single route-km of HSR are about €100,000 per year, versus €20 million for construction (see PDF-p. 9 of a study by Ginés de Rus about HSR between Stockholm and Gothenburg). With this amount of maintenance, there need not be any closures or disruptions in service.
Consider the Northeast Corridor, more concretely. To guarantee redundancy everywhere, so that train accidents do not disrupt the line, is to restore some passenger service along the former Baltimore and Ohio and tie-ins. Between Philadelphia and New York this means the West Trenton Line; between Philadelphia and Washington this means the CSX freight line. This also requires new Hudson tunnels. The cost of each of these elements is in the billions, and for the most part, with the exception of the new Hudson tunnels the transportation benefit is very low, especially south of Philadelphia, where there aren’t enough people to justify a second commuter line. Between New York and New Haven, there are no good alignments for a second route except for short bypasses; that’s what makes constructing HSR there so difficult.
Redundancy is a good feature of networks where failures are frequent and unavoidable; for such systems, redundancy is useful, as is the concept of failing gracefully. Rail transit is not such a network. It is both possible and desirable to reduce accident rates to levels approaching zero. Natural disasters remain hazardous, but are extremely infrequent, and at any rate when a deadly earthquake strikes, there are higher priorities than providing alternative passenger rail routes.
This is not to say that redundancy has no uses. Dense subway systems are redundant in the sense of providing multiple routes through the city – although, at the peak, they’re usually all very crowded. This makes it possible to shut down lines off-peak for maintenance; New York and London are both notorious for weekend service changes, and Paris shuts down short segments of lines for maintenance for a few weeks at a time (see for example here). But small subway systems manage to make do with just ordinary overnight shutdowns, and Copenhagen even runs trains 24/7, shutting down one track at a time at night and using the driverless operation to run trains on single track. It’s just more convenient to have more options, but not necessary.
The upshot is that when a subway or mainline rail network chooses where to lay additional lines, it should ignore all needs of redundancy, except possibly as tie-breakers. The benefits are there, but do not outweigh the cost of building less optimal lines. The operator should instead invest in systems, worker training, and maintenance regimes that ensure high reliability, and expand the network based on ordinary criteria of expected ridership and capacity needs. There’s no need to worry about failure, and it’s much better to design the network not to fail in the first place.