So, a number of years ago, the country of Peru decided to modernize
their railroads. They had been using steam locomotives, but want to
upgrade to diesels. The old steam locomotives were sunsetted and the
new diesels took their place.

Only, they discovered that it took TWO diesels to pull the same number
of cars up the Andes as one Steam locomotive had been.

The reasoning is clear in retrospect. Diesels need lots of oxygen. The
Andes are some of the highest mountains in the world, and as you go up
in altitude there is less air pressure  (and hence less oxygen). The
Steam Locomotives, on the other hand, depended on BOILING water,
something that occurs at a lower temperature (and hence requires less
energy) at high altitude.

Modern upgrades aren't always better. Something to think about.