Please, do be careful with the altitude; I’ve had some horror stories.
Generally it’s considered that above 2400m things start getting interesting. It’s definitely worth acclimatising for a couple of days if you’re doing anything physical at that altitude.
However, as you point out, it’s a passive activity for a short period of time. Perhaps take some anti-altitude sickness pills or cocoa leaves beforehand, to help with any symptoms.
But as long as you’re not over-stressing your body, and don’t have anything too physical planned for the preceding or following day, you should be ok.
You can buy your tickets here. The price is about 185 USD.
It’s not necessary to do any special preparation, as the train will take you back to “normal” altitudes after a couple of hours at 4200m. When I was in Argentina, the train didn’t work, so we had to rent a car to do more or less the same path the train makes. We stayed about 2 or 3 hours at 4000 meters and started feeling a bit bad, nothing important, but I believe if we had stayed some more time we could have had some problems. We had headache for the rest of the day and the night, so I recommend not to plan any hard activity for the next day when you go.
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