What's on the rail for the lizard?

"'What's on the rail for the lizard?'

Anne had many expressions deriving from her Mississippi childhood. This was one I hadn't heard before.

'Translation?'

'What are your plans for today?'"

From "Monday Mourning" by Kathy Reichs (Scribner, New York, 2004).

ESC, as I've never heard this before, but quite like it, I'm just looking for the original literal meaning... is this based on what desirably dead insect life might be squished on a nice warm railway line for the dining pleasure of a lizard? I'm using my imagination here as you can see, but that's the image conjured up in my mind. Or is it some other kind of rail, or something completely different altogether?

The U.S. has fence lizards, often seen resting (or maybe they're working, it's hard to tell with a lizard) on wooden fence rails.

Oh OK. Yeah we have those guys too. I wonder if they all belong to the same union??