Marty Ross asked:
"Does anyone know when the last truss rod cars were built and for how long could they be commonly seen in active use?"
 
The last truss rod house cars built featured underframes with steel center sills.  The most notable example of late truss rod cars were the Southern Class SU 36 ft double-sheathed boxcars.  The Southern acquired over 15,000 cars between 1922 and 1928, and the cars ran through the 1950s.
http://www.steamerafreightcars.com/gallery/boxauto/sousumain.html

HO scale models are available from Westerfield.
https://id18538.securedata.net/westerfieldmodels.com/merchantmanager/index.php?cPath=37
 
 
Ben Hom