John, you remember correctly. There is one circuit that probably keeps the transformer from cooking due to continuous overload, and a second circuit for shutdown for a bad short.
Although I started out with some fast magnetic breakers, I must ask the question "Just what are you protecting?"
A derailment usually involves wheels, pickup rollers and wire, and during the derailment short the voltage across any electronics downstream drops to near zero. When the short clears, there can be an inductive voltage surge to the electronics if current is still flowing. If, on the other hand, the power has been removed by the protection device(s) before the short clears, the surge would presumably be across the protection device since the track is still shorted when that device opens.
If the protection is for the wiring, the wiring inside a locomotive will probably survive a quick jolt of current while even a "slow" thermal breaker or fuse is acting. The problem arises with a sustained partial short that doesn't trip the protection, cooking the wiring.
I use the rocker-style TE breakers (8 amp) for wire protection (and convenient circuit ON/OFF isolation) and TVS protection for voltage surges.
I used Polyfuses for a while, but I find the rocker breakers to be more convenient, keeping the power off while I clear the problem.
