I see this photo on Facebook tonight. Apparently in Mexico in the 1880s. What am I seeing?? The loco cab and two cars have additional "roofs" on top of the cab/car roofs...?? Is this to keep the heat/temperature down in the cars?

(Also of note: Loco is a 2-6-6 wheel arrangement)

I'd say that's a very good guess.

Looks like the assessment that the "extra" roof was an attempt to keep temperatures a bit more tolerable is correct.

What stands out in the link shown below is that not only was this Mason-Fairley built by Baldwin instead of Mason, but the "sun roof" came with the engine from Philadelphia!!

https://www.steamlocomotive.com/locobas ... ad=cm#3163

Looks like the assessment that the "extra" roof was an attempt to keep temperatures a bit more tolerable is correct.

What stands out in the link shown below is that not only was this Mason-Fairlie built by Baldwin instead of Mason, but the "sun roof" came with the engine from Philadelphia!!

https://www.steamlocomotive.com/locobas ... ad=cm#3163

They are on the principle of 'safari roofs'. I had one on my Land Rover and it works surprisingly, extremely well.

The construction is a usually white-painted sheet -- thickness really only determined by structure; mine was aluminum sheet with white paint -- held off the actual car roof with small-area standoffs, usually posts. There is no attempt to seal the adges; there in no 'ridge vent' to duct 'cool' air from edge to center (as on some '30s manufactured housing).

The principle is only partly to 'reflect sunlight' to decrease solar gain. The two roofs define a relatively trapped air space, with low convective transfer, and heated air is a good insulator.