John,
Although they are also available for the firm's 33' hoppers, Micro-Trains currently lists replacement stirrup steps for their 40' and 50' boxcars, flatcars, gondolas, stock cars, and wood reefers.
https://www.micro-trains.com/index.php?route=product/category&path=104_122#/stirrups-tags/sort=p.sort_order/order=ASC/limit=100/minPrice=6/maxPrice=20While they can probably be acquired by contacting Micro-Trains directly, to date, replacement stirrup steps for the firm's heavyweight passenger and troop cars have not been marketed as factory replacement stock numbers.
Note:The "traditional", one-piece, Micro-Trains metal frame boxcar chassis had four pins cast into them, which required replacement stirrups with a pair of holes cast into the flat cross-member of each step casting.
As the original models were retooled, contemporary boxcar chassis (i.e., those with the separate, injection molded plastic underbody subframe detail castings) have rectangular pins at each corner, which require stirrup castings with rectangular holes that are positioned above each step.
Not sure if Micro-Trains has ever offered replacement stirrup step castings for their contemporary boxcar releases.
Neville