The main structure is built using planed timber around 50mm x 25mm (minimum size) with 75mm x 25mm ideal size edge ways on to give additional strength so the top surface remains flat when weight is added to the top surface.
It’s a good idea to use a spirit level to ensure the top is level remembering train run best on a flat level surface.
The maximum space between the cross members should not be more than 30cm assuming the top material will be a minimum thickness of 6mm (12mm is ideal) ply wood or similar material.