DCC Concepts Liquid Lead Shot (500Gm)
Genuine lead shot is becoming nearly impossible as it is no longer available for shotgun use in most countries. Lead remains the best choice for adding weight to locos though as it is double the weight of the castings used in models volume for volume.
The fine size of the shot makes it easy to use so it is the perfect way to add loads of weight in a small or awkward space. 500gm (about 1.1 lb) is enough for many N or OO/HO locomotives. Add the pellets then secure with diluted PVA, varhish or superglue.
For a great way to see in advance how added weight may help loco perfomance also take a look at Weight Mate. Weight mate is a bag of the same lead shot in a soft fabric bag that easily mould to the top top the loco without falling off, and because the covering is soft, it will do it without damaging detail, making loco testing much easier. (and DCC programming track use more reliable because of added wheel contact).