Dating myself terribly, half a century or so ago MR used to publish plans and descriptions of “Dollar Cars” (to be scratchbuilt for a dollar, more or less.) I built several (a drop-end mill gon stands out in my memory) but none of them survived their first encounter with military contract movers.
In the early 1960’s I decided to model Japanese prototypes. Since then, MR has served me as a source for ideas and inspiration, but not for specific things to model or otherwise build.
The funniest one I ever heard of is from the Willimantic (CT) RR museum. They built a real 1:1 hand car (a true “gandy dancer”) using plans from Model Railroader.
The most extensive was a 6 car fleet of the Southern Pacific Double Stack 5 unit cars. The only commercial parts were the trucks, couplers, and brake details.
For the club, I built the Memory Walkaround throttles.
Only one or two from the pages of MR (a small wooden highway bridge and a bucket coaling station, both from 1950’s or 60’s articles) but quite a few based on material appearing in RMC over the years.