Where was this train photograph taken? I found it as a header on langsamreisen.de, but that page does not specify where it is. The filename contains the phrase
tren-del-sur, but I don't think it's at the 6.4 km Tren del Sur described on Wikipedia.
Assuming it's recent, it's almost certainly between Cusco and Puno.
Source? Well, it's circumstantial, but bear with me.
As it's open plains, I'd also plot it as between Puno and La Raya, if I had to guess, as the scenery changes in the second half. But I can't guarantee it.
However, there USED to be a train down to Arequipa as well, now I believe running only on demand for large groups. So if it's a stock photo, it might have come from that journey as some of the landscape would be similar. But it's a lot less likely.