let's assume it is above sea level.
Agreed - I think that's what was meant; it agrees with the information I can find about the line between Lima and Huancayo (which is the high point you mention), on the service called the Tren de la Sierra
The height is such that people often succumb to altitude sickness on the way, and in modern times there is a law that requires trains on the line to carry a nurse and oxygen, to care to for flagging travellers.
According to my sources, passenger trains outside of the U.S. in 1950 usually went at least 100 miles per hour.
Nothing like, I'm afraid. For one thing, it would have been impractical for most services - 100mph/ 160kmph is fine for long-haul, intercity trains under good conditions; wholly impractical for the majority of "stopping" trains, where stations may only be a few miles apart. And there is also the not inconsiderable matter of the terrain the train must travel over...
So the notion that the minimum
rate of travel anywhere in the world was ever "at least
100 miles per hour" (my emphasis) seems to be highly doubtful, and not even practical as an average, when you consider that the The Mallard
, which set the high speed steam record reached just a fraction over 125mph (just over 200kmph), and that was under specific "racing" conditions, not as part of a regular service.
We could consider this slow speed accurate because a train conductor would want a slower speed to stay safe, as he is going up a mountain.
However, the biggest misconception you have, I feel, is that a train - and a steam-train at that - could ascend three miles, on highly meandering track, through tunnels and over trestles at such speed: it assumes a prodigious effort on a virtually continual gradient, and that's before
working in the effects of altitude on combustion and water-boiling. Even today the diesel
-pulled trains take 12 to 14 hours to cover the 108 miles.
I couldn't find a reference to it for the Lima line, but in looking for information I did see a mention to the effect that in Argetina, on one of the mountain lines there, progress by the train is so slow at times due to the geography, that passengers can safely get off and walk along-side the train to stretch their legs!
There is a clue in the text: if they had been on the train for "several hours" - which is a term for more than a "couple of hours" (which would be two) - and they still hadn't reached the terminus, which would be the highest point, then the train is indeed going slower than even your lower estimate.
This also means that even after "several hours", the Captain and Tintin have not reached the full three miles high, so you are quite right to point out that the air will be thinner, but perhaps not as attenuated as you first thought?
There's nothing to say that the incline between point A and point B on the line is a straight graph, but if we imagine it is, and say that the train has been under-way for perhaps four hours out of twelve, they are only a third of the way there, so approximately a mile high?
Using this graph, the air pressure from where Tintin is jumping is about 50% of the air pressure at sea level.
I'm not sure of what that graph is showing me, to be honest, or exactly how to read the scales.
However, a mile high is the about same altitude as Denver, Colorado, and the atmospheric pressure there
is said to be about 50% of sea-level atmospheric pressure in the literature I can find, so we can agree on the figure, if not exactly how it was derived!
At that height, the air would be too thin to cushion Tintin's fall.
I think it's fair to say that, even at sea-level, air resistance would have very little effect on Tintin, at least in terms of being great enough to ensure his safe passage to the ground - so at altitude, under "ideal" conditions (still air, no wind, no buffeting from surroundings, etc.), it would have even less effect.
However you might
need to factor in prevailing conditions at Tintin's location - such as the effect of wind in an enclosed environment like the ravine into which he's jumping, which might increase pressure above that of standing air at a similar altitude; can't say I think that the outcome would be much different (i.e. pretty disastrous for our boy reporter and his dog), but in the interest of completeness we should at least consider