need some help

Before adding weight to a brass steam locomotive for added traction I first check to see if the leading and trailing trucks are inadvertantly "unweighting" the drivers.  This can be due to verticle clearance issues or overly tight springing. One test is with the locomotive on the rails you should be able to lift the leading and trailing truck wheels off the rail head without the drivers losing contact with the rails.  If the springs are too strong usually you can reduce the strength by cutting them a little shorter, or by relpacing them with a softer one.  Clearance issues need to be addressed on a case by case basis - sometimes with some careful grinding with a Dremel tool in areas that won't show.

If springing is not the issue, then adding weight is the solution.  I  add weight one of two ways.  Either by sliding pieces of lead around the existing weight, or casting a replacement larger one. For good tracking the center of mass should be approximately over the center of the driver wheelbase.  

Ed Rappe

If you are using kadees on the locomotive and they are low try a set of the 747 Kadee underset shank couplers. I've used the overset shank type on some 17/64ths scale cars I have and they brought the height right to standard.  First thing to try before getting into modifications.  

http://kadee.com/htmbord/page747.htm