Trip speed is the key, on Digitrax system, the booster trip speed is configurable to work with downstream protection devices. Assuming a dead short, like a reverse loop, you could have an autoreverse device set to 5 amps running off a booster that only supplies 2 amps and have the reverser trip first - if it is set to a faster trip speed. In the case of a not so solid short, like a derailment that leaves a metal wheel across the track with little weight on it, the trip current might take precedence and the booster would trip before the circuit breaker. Both things need to be taken into account for reliable operation of circuit breakers and autoreversers, but it is possible for a breaker that is set for more current than the booster can deliver to still function.
–Randy