Anthony, have you looked at the Lionel Powerhouse 180 circuit protection? It has exactly what you mention, overcurrent and spike current protection.
In the schematic, both U1b and U1c are amplifying the current sense signal. U1c, the "overload" detector, has a gain of 148, but it is slowed down by the resistor and capacitor on its output. The time constant (RxC) is 2.6 seconds. Multiple short hits to this RC combination would charge it up until it trips the relay latch.
U1b has a gain of 37, which means it requires 4 times as much current, but it acts instantaneously for "dead short" situations.
