Case in point is a 3D printed wall train created by the artist Can Altinellar.
Altinellar's vertical engine had a number of technical challenges that had to be overcome in order to defy gravity. Magnets and rubber coated wheels enable the locomotive to stay on track, but one could assume the actual construction of the train, if not for 3D printing, would have been a barrier keeping Altinellar's vision from being realized.
Alitnellar has posted the STL files for his train on github for those who want to replicate the project.
Anyone who has ever shelved an innovative product design fearing the cost or complexity should take a mental ride on Can Altinellar's vertical train. The journey may provide the realization that once discarded ideas can be reborn when you combine ingenuity, perseverance and 3D printing.