Please find attached a file that includes a RailClone object with a macro for this style (requires RailClone 3). In future version we will include this in the built-in macro browser. It was created by alternating a small brick and a large brick in the Left and Right inputs. Padding was added to both brick type that's the X size of the large brick minus 2 x x size of the small brick. If you look inside the macro you can see the setup, it look a little more complex because I've added a few other features and made it so that all the dimensions are extracted from the geometry automatically.
I hope that helps, many thanks!