I know that this is not a dumb question asked on a mine tour, but I couldn't resist retelling it....

Overheard in a dome car on a steam excursion behind Milwaukee Road 261 and apparently, this guy's kid is a big railfan, so he was along because the kid wanted to go-

"So is this a diesel?"(!!!)

My dad and I were blown away.

I truly thought that this guy's kid was going to die of embarrassment! My dad said "Dude, it is called a STEAM LOCOMOTIVE. Sure, some of them ran on fuel oil, but it is not a diesel."