Manufacturer Rolling Stock Size Limits? Test Car?

Don, I think the closest thing you’ll find is the National Model Railroad Association (NMRA) O gauge tool, but as so many manufacturers don’t build to scale exactly, your best shot is to adhere to recommendations for each layout. For instance, Lionel makes models in scale, semi-scale, O-27; and MTH makes scale, Rail King (which can be scale, or O-27; and others, such as Marx, RMT, K-Line, etc.

On my own layout, with O-42 curves, I found I couldn’t run  piggyback cars through my Lionel tunnel portals on my outer oval, but they run fine on my inner oval, which has better clearances. My next expansion will allow for more generous clearances.

Every model has it's own requirements for overhang.  A Big Boy, 86' High Cube and 80' passenger car will all have different overhangs.  It would be impossible to make one clearance car to fit all model scenarios.

Even it the prototype world, not eveything can go everywhere.  SP 4449 wouldn't clear the platform's at Chicago's Union Station for example.  Nor could it make (a curve if I recall correctly) it to display at Navy Pier in 1976 with the Freedom Train.

Rusty

It would be great if time would just stop and allow us to catch up.

Train car sizes have changed over time. Same is true for our model train cars.

I have made Clearance Cars from acrylic rectangles  with trucks attached having vertical inserts to represent the height and width of the largest cars usually the newer autoloaders.