The simplest way to think about an in line console is a console where they have taken two channels and stacked them on top of each other. So a 24 channel in line console would actually be a 48 channel board but with the second half stacked on top of the first.
This is not technically exactly correct especially since the second set of channels has some different features etc, and the rounting scheme is more involved than I have just stated, but its an easy way to warp your head around the concept.