Okay, this is what I've come up with - a discussion of the meaning of "smokestack lightning", where someone said:

This was based on (Howlin') Wolf's "Crying At Daybreak" recorded years earlier and itself modeled on Charley Patton's "Moon Going Down". The inspiration, said Wolf, was watching trains cut through the night:

"We used to sit out in the country and see the trains go by, watch the sparks come out of the smokestack. That was smokestack lightning."

I still need to find where they got that information though. Anyway, I was just talking to John about it. He's a steam train buff, and he just showed me some rather startling pictures of sparks coming out of the stacks of early wood-burning locomotives, and particularly the locomotives in the cane-fields that used a by-product of sugar cane, called bagasse, as fuel. Because it was coming out under pressure, the sparks danced around and looked quite a lot like lightning. I'll see if I can find some pictures - meanwhile I'll post this.