A ram accelerator is a strange beast: a projectile shaped like the centerbody of a ramjet flies down a tube pre-filled with a premixed fuel/oxidizer gas. The projectile's own bow shock compresses the gas, ignites it behind the shoulder, and the resulting detonation pushes the projectile forward. No moving barrel parts, no plasma armatures — just chemistry chasing a bullet. UW-Seattle's lab has hit ~2.7 km/s with pound-scale projectiles. What if we scaled it up and buried one between Chicago and St. Louis to fling freight pods?

The setup. A 500 km buried tube, 2 m bore, pre-charged with segmented methane/oxygen mixtures at 5–50 atm. A 2-tonne pod (steel, tungsten nose) enters at Mach 3 via a light-gas gun pre-launcher, then rides staged detonations to Mach 8 (~2.5 km/s). It coasts most of the route in an evacuated section, then decelerates in a reverse ram section that extracts energy back into the working gas.

Energy budget. Kinetic energy of a 2000 kg pod at 2500 m/s:

KE = ½ · 2000 · 2500² = 6.25 × 10⁹ J ≈ 1740 kWh

At $0.10/kWh electricity equivalent, that's $174 of energy per pod — cheaper than the diesel to truck 2 tonnes 500 km (roughly $200 in fuel alone). Trip time: ~3.5 minutes at cruise, vs. 7 hours by truck. A pod every 30 seconds gives ~240 tonnes/hour, comparable to a busy freight rail corridor.

Where physics starts pushing back.

• Stagnation heating. At Mach 8 in a dense fuel mix, the nose sees stagnation temperatures near T₀ ≈ T·(1 + 0.2·M²) ≈ 4400 K. Tungsten melts at 3695 K. You need ablative or actively-cooled noses, and each pod loses maybe 5–10 kg of material per shot. Refurb between flights isn't optional.
• Tube pressure integrity. Detonation peaks hit ~200 atm behind the projectile. A 2 m bore at 200 atm needs wall hoop stress σ = P·r/t. For steel at 400 MPa allowable, thickness = 200·10⁵·1 / 4·10⁸ = 5 cm of steel along the entire 500 km — that's ~6 million tonnes of pipe, roughly the steel in 700 Eiffel Towers.
• Gas logistics. Each firing consumes the fuel mix in the accelerating section (~50 km). At 20 atm average and 2 m bore, that's ~3 tonnes of CH₄/O₂ per shot. 120 shots/hour = 8600 tonnes/day of premixed reactants. You've basically built a rocket-propellant plant next to every launch tube.
• Seismic coupling. Each detonation dumps ~6 GJ into the tube walls in under a second — a magnitude 2.5 event. Fire one every 30 seconds and you're a persistent local earthquake source. Isolation mounts along the entire tube become the dominant cost.

The verdict. The energy math is startlingly favorable — kinetic transport is intrinsically cheap because you only pay the KE once, not continuously against rolling friction. The tube itself kills it: you're building a 500-km-long pressure vessel that also functions as a detonation chamber. Rail moves cargo at 0.02 kWh/tonne-km using 150-year-old technology. The ram accelerator moves it at 0.87 kWh/tonne-km but 100× faster. It's a courier service for things that must arrive in minutes — organs, semiconductor masks, maybe munitions.