Resistance in series with a toy-train motor, a traditional way of controlling the train’s speed by reducing the voltage, will not harm the motor. But when you try to cut something with the circular saw at a low voltage and turning slower than it should be, it tries to cut too much wood with each rotation and so draws more current than is good for it. The train on the other hand needs less torque and therefore less current as it runs slower, protecting the motor.
Of course, if you have too much circuit resistance, you can’t get the train to go very fast; but you won’t damage it either.
I have often advised connecting headlights to motor brushes on the forum. It works great for directional lighting, is easy to do, and easy to undo if you change your mind later.