Oops, forgot - Squires tools used to sell "liquid lead" which was jars of incredibly fine lead shot (or some lead substitute) that could be poured into any suitable spaces on a chassis or bodyshell and sealed with epoxy resin.
I bought some from them at a show years ago, probably Bristol show, and still have loads left as it goes a really long way so that might be an avenue to explore too if you need to fatten up your loco's.