Circuit Breakers are the most important Marx accessory. These saved countless derailed and shorted engines. Unless you have had a derailment that went unnoticed for a few minutes you probably don't know how quickly the shorted circuit can heat up to fire danger levels. These inexpensive breakers are very reliable and easy to find. Use one on every circuit.
The bullseye signal in the center is not a breaker, it is made from the same body but is simply a light.