* trains are long
* trains travel slower than airplanes, and drag is proportional to the square of speed
* making conclusions solely based drag coefficients from the density of the air is sound reasoning
One of these statements is false. I get that hackernews is just a forum for casual discussion of various technical topics, but overly categorical statements based on a reasoning in a vacuum (no pun in intended) just force the conversation into unproductive threads like this one. I'm kinda sorry I took the bait at this point.
I hear this sentiment a lot, and I don't understand it. Trying to speak the truth isn't just optional outside formal contexts. There's no situation so casual that I it suddenly becomes appropriate to make up bullshit and present it as facts.
That is to say, I agree with your larger point: if nickff wants to make the claim that trains consume more energy than aircraft, he'd better come up with a reasonable source for that information.
I think if you want to make this analysis come out you need to include starting/stopping losses (and maybe fuel hauling, empty car overhead, etc...) to the train trip or somesuch. Naively trains are going to win and it's not even close.