I use 'Double OO" bird shot that I get from my local gun shop. I mix it with PVA glue then paste it into whatever space I have to fit it in. I also melt it down, or fishing weights, to use in loco chassis that are a bit light. I mill out parts of the chassis and fill the void with the molten lead. As most of the chassis for modern models are made from a zinc alloy the chassis has no problems handling this.