Well, the power light is always lit, even when the breaker trips. OTOH, the output should obviously not still be at 18 volts. 
The breaker logic in those is a little odd, the relay actually is activated when the breaker is tripped, and it inactive when track power is flowing to the track. That being the case, my guess is something in the circuitry has failed, there's a loose wire, etc.
As you can see below, there's quite a bit of circuitry to do the circuit breaker function in the PH180. Click on the schematic for a full sized image.
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.
