Technical debt in a rail system

Last updated on July 18th, 2021 at 05:18 pm

Acela Express rounds a curve in Connecticut
Acela Express rounds a curve in Connecticut. Shown is the trailing power car of a southbound Acela Express and the front of a northbound Metro-North railcar.

Most definitions of technical debt require that the asset bearing the debt be software. From the policymaker’s perspective, this requirement is rather limiting. So for the purposes of this blog, I define technical debt as any property of a technological asset that we would like to revise, replace, or create, and which limits the ability of the enterprise to gain or maintain a market dominance. (See “A policymaker’s definition of technical debt.”)

An example from the railroad industry

In the United States, the highest-speed rail line is Acela Express. Acela travels in the northeast corridor between Boston and Washington, D.C. Parts of the right-of-way, track, and catenary it uses are from legacy lower-velocity applications. That’s why Acela trains cannot operate at their highest possible speed [Maloney 2000]. On the 231-mile section from Boston to New York’s Penn Station, Acela achieves an average speed of only 63 mph (101 km/h), even though the trains can operate safely on straight track at 150 mph (240 km/h). Yet, Acela still manages to capture a 54% share of the total air and rail market between these two cities

How Acela’s technical debt slows its trains

That 54% share might be higher still if not for technical debt. To compensate for centrifugal forces as Acela rounds curves, its passenger cars tilt their passenger spaces. The tilt enables the train to round the curves at higher speeds than would otherwise be comfortable for passengers. In effect, the cars “lean into” the curves, just as a running athlete leans when rounding a curve. Although the cars could tilt by as much as 6.8º, the adjacent set of tracks is too close to permit this without risk of collision with trains on those tracks. The maximum permissible tilt in this system is therefore 4.2º, which reduces the maximum speed consistent with passenger comfort that the trains can attain on curves. The technical debt in the tracks Acela uses thus prevents Acela from offering a service that would be more competitive with alternative transport modes, especially airlines.

Last words

In August 2016, Amtrak announced that it will be upgrading its trainsets and tracks to exploit new technologies, including active tilt technologies. All existing trainsets are due to be replaced in 2021-22.


[Maloney 2000] Brenna Maloney and Don Phillips. “All Aboard AMTRAK’s Acela,” The Washington Post, November 30, 2000.

Available: here; Retrieved April 18, 2017.

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