In a new blog post, noted Hyperloop critic Alon Levy lays out the case that shipping freight is not a viable use for the theoretical technology, thanks to a confluence of technical and business constraints. Freight shipping has been the focus of Hyperloop One, the most advanced and well-funded company working to build a Hyperloop.

Levy, a mathematician and prominent voice in transportation and infrastructure debates, points out that the sort of heavy freight now carried overland by trains would not be a good fit for the Hyperloop, because the lateral force generated on curves would overstress the system’s proposed light elevated pylons. That would limit potential uses to freight that is more valuable than it is heavy, such as jewelry, mail, and perishable goods.

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Levy argues that even those limited uses, though, are hamstrung by factors including the so-called “last mile” problem. Because the Hyperloop would travel between fixed points, trucks would still need to transport goods at each end. With that reality taken into account, Levy writes, “any speed benefits coming from high-speed freight services vanish.”

Given those limitations, the Hyperloop seems most primed to compete with air freight services, which currently cater to the light-but-valuable freight niche. Hyperloop advocates argue it would combine plane-like speed with rail-like efficiency and low cost.

But the Hyperloop faces further fundamental disadvantages when compared to air freight. Most significantly, the air shipping network is highly flexible, able to frequently shift routes to match the demand for moving high-value, low-weight goods. But with its fixed tubes, the Hyperloop would not be able to alter its routes with shifts in the market.

Moreover, the air freight industry’s own revenues have declined significantly since 2011, suggesting that there’s little broad demand for a new competitor in the high-value freight niche.

Hyperloop One has used the freight thesis to position itself as a sober, serious company pursuing grounded applications, rather than simply building a niche product for wealthy travelers. In a New York Magazine feature detailing the company’s recent leadership turmoil, former Chief Technology Officer Brogan BamBrogan said the focus on freight helped convince him to join the company. It has also helped Hyperloop One raise over $160 million in venture capital.

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But Levy is not the sole skeptic of the freight proposition. Supply chain professor Barry Prentice made similar points when speaking to Inverse in May. Because passengers generally place a much higher premium on speed than do freight shippers, Prentice argues that the sweet spot for the Hyperloop might be to copy commercial airlines’ model of mixing passengers and small amounts of freight.

Levy has been a persistent skeptic of the overall Hyperloop concept, first introduced by Tesla CEO Elon Musk in 2013, and may even have played a role in Hyperloop One’s emphasis on freight. Shortly after Musk’s initial Hyperloop whitepaper, Levy’s calculations suggested that a passenger Hyperloop would both be far more expensive than Musk’s projections, and produce enough G-force that it would be, in his words, a “barf ride.”