In an effort to do something about my huge mountain of unbuilt kits, I've assembled a handful of US boxcars today. These older 'shake the box' kits from Athearn & Model Die Casting are much simpler than their UK equivalents and typically take about five minutes to throw together. All of my US stock runs with Kadee couplers and wheelsets as standard, so these newest additions have been similarly equipped.
The supplied steel weight is discarded and replaced with something a little less inclined to make a sprint for the nearest uncoupling magnet. I weight US stock according to NMRA guidelines, which gives a suggested weight of 4.5 ounces for a 50 foot boxcar, quite heavy by UK standards. It probably takes longer to fit weights to these cars than it does to build them....
My preferred method of weighting for stock such as this is to use self-adhesive window leading, sold on a roll by various D.I.Y. stores. It's quick and clean, but above all has the advantage of being flexible. This is a major plus point when stuck to a floor that has to be bent to be inserted into the body. I've got a number of items of rolling stock with loose weights rattling around inside, a consequence of the stress put on the glued joints when bending the floors, but I've never had that happen with the self-adhesive strips.
I use other weighting methods where appropriate (I'm a great fan of 'liquid lead') but for a lot of US stock, as well as British coaches and multiple units, I've yet to find anything to beat this strip.