Small truck rail riding carriage. 9:30 video:
https://www.youtube.com/watch?v=DXc9DbEy-1M
Previously:
https://www.homemadetools.net/forum/...783#post140169
https://www.homemadetools.net/forum/...075#post134237
https://www.homemadetools.net/forum/...061#post134201