I built my own railer for getting stock on the track. This is a top view. The ruler is for those who want to copy the design.
The black strip is raised and is straddled by the wheels which are guided onto the track at the right hand end.
Although it works well enough as illustrated it is improved slightly by tapering the guide only along the section where the white part is wide and making it parallel (13.5mm wide) along the length of the tongue that sits between the rails.