The stock breakers in the AH601 are thermal. That means that they take time to heat up when there's a short circuit condition. The instant breakers I linked earlier trip in milliseconds. If you go with one of the instant breakers, either a 7.5 or 10 Amp would be my choice for the AH601. The 5 Amp would likely buzz with a heavy load.
Also, seriously consider adding TVS diodes for your modern locos if you haven't already.