Electric circuit breakers are great, but the ones built into power bricks and transformers are designed to protect the transformer. It is still advisable to have discrete breakers on each circuit/channel appropriately sized for the expected load.
A loose corollary would be like your residential electric panel. Your power supply has a main breaker (100AMP or more service) that could be seen as the built in breaker of your train transformer. However, each circuit in your home has a discrete breaker sized for its intended load (15 AMP). You should do the same on your layout for discrete track blocks, accessories, lighting districts, and whatnot
And it is worth repeating that these breakers do NOTHING to protect against the transient voltage spikes discussed above, so TVS diodes are still a complimentary necessity.
