[mapt]

Far easier than an evacuated tunnel, though, is a tunnel built on standard-pressure hydrogen/helium. Light gasses have a much higher speed of sound, and their lower density means lower drag. A system of hydrogen-filled maglev tubes that can max out at 2500mph with freight-weight loads over long hauls would be more than a little bit valuable.

[dpark]

A tunnel filled with hydrogen seems to be a very bad idea.

http://en.wikipedia.org/wiki/Hindenburg_disaster

A tunnel filled with hydrogen has a large surface area for potential leaks.

[mapt]

I don't think I need to debunk what happened to the Hindenburg yet again (and, incidentally, what didn't happen with it, its sister-ship the Graf Zeppelin II or its ancestor the Graf Zeppelin I in 682 other flights).

Suffice it to say, the challenges of an electrically grounded hydrogen tunnel lined with heavyweight materials using modern technologies & sensors are a hell of a lot different than the challenges of an ultra-lightweight, ultra-thin flammable gas bag before the advent of plastics, in an electrical storm.

A tunnel filled with vacuum has it a hundred times worse, because you're dealing with 15PSI trying to get in rather than 0PSI pressure difference diffusing out. We already regularly use larger-molecule methane at thousands of PSI.

[dpark]

I'm not sure what exactly you mean when you say "debunk what happened to the Hindenburg". The thing became engulfed in flames and crashed. Not much debate on that. Or are you talking about debunking the claim that hydrogen airships are fundamentally unsafe, a claim I didn't actually make? (Incidentally, the fact that other hydrogen airships didn't suffer the same fate doesn't mean they were actually safe. Lots of homes have aluminum wiring but have managed to avoid burning down. You'd still be wise to replace any aluminum wiring you find in your home.)

Obviously a modern hydrogen tunnel would be much different than a zeppelin, and have different challenges. Still, 400 miles of 10-foot diameter tunnel would have plenty of opportunities for problems (whether it's evacuated or filled with hydrogen or something else). I'm not sure an evacuated tunnel is necessarily worse from a safety standpoint, though. It might be harder to engineer, but the failure mode seems safer. A hydrogen tunnel means that a leak will mix hydrogen with an oxidizer. If it's at 0 PSI, this could mean a leak lets oxygen into the tunnel (either due to high-pressure weather or due to hydrogen rising from the leak, leaving a low-pressure environment in the tunnel). If you wanted to fill a tunnel with hydrogen, I think it would be wise to keep at least a small positive pressure to ensure than any leaks are outward only.

I'm totally unclear why methane stored at thousands of PSI is relevant. Is there some 400-mile long, 10-foot diameter, above-ground methane tunnel I'm not aware of?