With the table saw, I cut four 6-foot long pieces with a width of 1 1/8' for the tracks (the height is given by the thickness of the 3/4' board). With a marble size of close to 3/4', I decided to make the track 7/8' wide with 1/8' wood on either side, adding to a total width of 1 1/8'. I had a number of glass marble lying around with slightly different diameters, around 17mm (or 11/16'), so I had to be careful that the individual elements of the track worked for a range of different sizes and weights of the marbles. While each square in the sketch corresponds to 1/4', please note that these are only approximate sizes.

I just started with the frame and the above plans for the marble lift, and improvised each further step. That's why I do not have a perfect drawing with the final measures. I did not plan the whole project in advance. So, in every cycle in which the front piece is lowered and then raised, all marbles walk up one step. When moving piece is lowered again, the marble rolls onto it, and then to the right (4). When the moving piece is raised (2), the marble rolls backwards onto the solid piece and then to the right (3). When the moving piece is lowered, the next marble enters (1). Furthermore the stairs of the static piece (which is sitting in the back) are tilted toward the front, and the stairs of the moving piece (which is sitting in the front) are tilted towards the back. The stairs of both pieces are slightly tilted to the right. A static piece (the blue one in the sketch) and a moving piece which slides up and down on two wooden dowels (red indicates its lower position and orange its upper position), operated by a servo. The marble lift is made of two stair-shaped pieces. The marble run is built into a frame of size 16'(h) x 20'(w) with a depth of 3'.

John lewis wooden marble run