I'll venture a guess...

Back in the 20's when alot of these preward songs were recorded transportation wasn't what it is today... Bummin' rides was a way of life for alot of folks, especially poor black folks and especially poor black musicans... There seems to be this connection between the folks who helped these folks get around and lyrics in blues songs... It's almost an "owin'" to those who helped with transporatation...

The best known line from those days was "ride the blinds" which meant that a railroad conductor would allow folks to ride behind those canvas curtains between the rail cars... They were called the blinds...

Oh, depot agaent
Please, please let me ride the blinds
I wouldn't mind, Son
Bu the New York Central, she ain't mine... (Son House)

But there were plenty of folks out there who must have let alot of folks on the trains, as long as these folks didn't sit in the rial cars, use trains for free transportaion... This, I think created this "owing" relationship btween the rider and the the person allowing the rider free transportation...

But alot of the blues stuff we really aren't all that sure of and this may fall into the category... Might of fact, alot of the blues stuff is just enuendo kinda stuff where words take on much differrnt meanin's when used in a song... I mean, just how something as common as "jelly roll", a pastry, ended up being used to refer to an anatomical part of a women is kinda funny... Kinda cute, too...

B~