Early on after Frank die cast his E coupler,the question of strength came out-this was at least 2009. A couple of modelers tied a cement block to a coupler somehow as I remember,& the Sergent didn't fail.
I plan on 20 car trains,maybe 25 as I have Atlas diesels & that's what I was told an engine could pull per unit not in lash ups. So I figure I'm ok,but I have to remember,even if a knuckle fails,it happens on the prototype as nothing is 100%.
Your question is a good one & the answer might still be listed under,"Frequently Asked Questions," at the Sergent home page.
Hope this helps.