You don't need a slot that was manufactured in as if they did that then they would have added a weight themselves. No, all one would need is to open the shell and glue a weight in. I have done it on many trains including a Shinkansen that had a tendency to do a wheelie on the top of slopes and derail. I like to use a clear Silicone Caulk as glue because it holds well and if I want to remove the weights the caulk is easily removed.
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