I put a candy thermometer in the cab of our engine, because a regular thermometer doesn't go high enough.
The typical reading on a hot summer day - maybe 95 degrees outside - is 140 to 145 degrees inside the cab. It's brutal. I will drink several quarts of water and Gatorade over the course of the day, but won't need to use the restroom.
When we shut down our engine, we put a cap over the stack to keep the heat in. All that heat that was going up the stack now creeps back into the cab. When we're done, we also shut all the windows and pull the rear curtain. It gets REALLY hot then.
Unfortunately, a couple minutes after I shut and locked the door at the end of the day, our engineer realized he had left his cell phone (turned off) inside the cab. He went in to retrieve it. When he came out, I asked if he checked the temperature. He responded that he had, and it was 210 degrees! That's two degrees below the boiling point.
Yep-the cab of a steam engine can get very hot indeed.