The straight line walking mechanism based on transfer the crank shaft rotation movement to the sliding linkages to make the foot part moves in straight profile to make it stable and balanced for all cart.
All files for the 3D model are available and you can download and fabricate an education model, all parts able to modified as per your size and application.
for more details about the mechanism desgin please review the following article https://erobtic.wixsite.com/erobtic/post/straight-line-walking-mechanism