San Juan evolution and Protocraft couplers uncoupling due to length and weight. Limits?

"Dave,what switched you from O scale to S or possibly HO?"

   Hi Alan, Mostly it was a size decision. I had plenty of O scale stuff and a big room to build a layout but as I neared retirement I decided I probably don't want to maintain such a large place so I'm building something that can fit into a smaller space if I decide to move. S is enough smaller than O to make it feasible and HO would be very simple to fit into my smaller layout space. HO has a lot better product selection but S has most of what I'd like so the choice is difficult.......DaveB

That all depends on the point.  In my case it is to drag 100+ bathtub gondolas through a flood loader and fill each one with real coal while the train is moving at a steady speed while getting heavier with each load;  just like the prototype.

Then you're probably a good distance off the bell curve on size of trains, space, and might experience the results of exceeding expectation by potentially exceeding the capabilities and limits of the equipment - 3 lbs / var probably exceeds operational recommendations and lead to failure of couplers that were never designed for that scale of operation being far removed from average.  Maybe metal couplers are called for just like the prototype.