Dan-Tuyet Tham

I've heard it's essential at altitude (so they serve it on trains).

I've never experienced altitude sickness but I was skiing with a
friend and his son near Santiago and his son looked very poorly
at the peak so we got 200m lower as fast as possible, where the
boy started to look a little better. It looked like an experience
very much to be missed!

Yes, from real coca leaves. It tasted like herbal tea. You do not get high on it. It should help on the high altitude sickness but I got that sickness anyhow and it lasted for 12 hours. That was terrible. A Powerrade helped. The tea worked more laxative on me.
I tried later on to chew the coca leaves with something grey, I think it was charcoal. I did not get high only numb in my lips and cheeks for 20 minutes as I recall it.
You can see it here:
http://www.23hq.com/Dan-Tuyet/photo/8638066?
You can also buy coca bon bons and it tastes like candy. I did not dare to import it to Denmark even though you do not get high on that, too.