The key thing to know when adding weight to any locomotive is the maximum rated operating current for the motor. Then you add weight until the locomotive starts slipping its wheels when the current is just UNDER its maximum current rating.
Here's how you would do this, and note that you must have a means of measuring the current going to the locomotive. If you have a transformer that has an ammeter built in you can read the amp draw on the transformer. If not, you'll need to have some means of measuring the current draw of the locomotive.
Let's say the maximum rated current for a locomotive motor is 3 amps. Put it on the track and block it so it will not move. Then slowly increase the voltage until the wheels slip. You want to know the maximum current JUST BEFORE the wheels slip, not the current draw while they are slipping. Once the wheels start slipping the current draw will drop, so it is critical to know the current drawn just before the wheels start to slip.
In this example, let's say the slipping current was 2 amps. You could add weight until you get to that 3 amp limit. By weighting a locomotive this way, you have a built-in fail safe. You know the wheels will start to slip before the motor exceeds its maximum rated current. You'll never burn up a motor using this method.
I don't know what the max current rating is for the motors in your locomotives, but others here may know.
