I have several documents here that will prove to you that I am a know-it-almost about electricity and electrical controls.
You do not need to make the situation worse by name-calling. It didn't bother me in grade school, and it doesn't bother me now.
If you would look through the archives of the electrical forum, you would find several posts by myself and other people who DO know quite a bit, and to a man, we discourage people who know little about electricity to try dangerous experiments when the science already exists to do it in a way that will not cause your skin to burn off, your eyes to blow up, and your house to burn down.
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Please, enlighten me as I know of no other way!
Now, to your question:
The answer is to put a load (not a short) across the output. The term of art is "load bank" and you can Google it. It can be a variable load, or it can be a load done in steps. The easiest arrangement would be a bank of several 40 watt light bulbs in surface-mount porcelain sockets, fastened to a 1 by 6 board, arranged in parallel, such that you could screw one in at a time, increasing the load. I have one here that I use to test telecommunications power supplies. The load would need to be monitored with both a voltmeter and an ammeter. Using Ohm's law, you would use a voltage of 20 volts (no need to crank the throttle all the way up) to do the math. Power (Watts) equals the product of amps times volts.
Current draw is arithmetically cumulative. A 40 watt lamp at 20 volts will draw around 2 amps. Add 2 amps when you screw in the next lamp. Watch the ammeter and see at what point the circuit breaker opens. Please be advised that this is an extremely primitive and simplistic approach, since lamps rated for 120 volts will not always behave according to strict rules when operated at lower voltages, but the principle is sound for this application. The key is to use an accurate digital ammeter.
Since toy train transformers, especially really old ones, might have components whose tolerance is wacky, I would take the advice of many other of my friends here, and use fuses instead of the 75-year old circuit breaker. Circuit breakers rely upon HEAT (and more importantly, TIME) to operate. In a cold room, with high heat dissipation, a circuit breaker will take longer to clear than in a warm room. There are several other variables, but that is the basic story.
You also need to acquaint yourself with a table of ampacities of copper wire, so that you do not inadvertently cause a test lead to become a fuse. That would also be dangerous and counter-productive.
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