Ben Hom wrote:
Plan ahead. Figure out what will and won't be visible when the car is on the track to determine where you can hide weight. To echo Richard Hendrickson, no point in modeling what won't be seen, and not modeling underframe or brake details in exchange for accommodating weight is a fair trade if you want the car to operate well. Try to maximize the amount of weight for the volume available - if you can't cut pieces of sheet lead, consider using the smallest shot you can find. (I'm not sold on more exotic solutions such as titanium dust as the cost doesn't seem to justify the incremental gain). Consider using thin sheet brass or lead under the deck if possible, and metal trucks if the correct type is available. Don't wait until the car is painted and decaled before deciding to add weight to the car (or work coupler and truck issues for that matter).
Lead is significantly denser than brass. Titanium is rather less dense than either. I agree with Ben that lead is an excellent choice, especially in sheet form. You can readily buy this from roofers' supply stores (sometimes plumbers also) even if your local Big Box no longer carries it. And an advantage of lead is that it is so soft, you can readily "forge" pieces to fit where they need to go in an under frame. You barely need a hammer.
There have occasionally been outbreaks of hysteria about the health dangers of lead. Well, lead in metallic form is not dangerous. It does have lead oxide on its surface, so do wash your hands after handling it, but beyond that, not to worry.