I started with the premise of controlling both sides of each brick and not making any other connection. This is the most flexible but depending on how common is handled SPDT relays could be used to break only the hot side. The control relays must handle the full current output of the bricks. The sense relays only need to handle the combined current of the control relay coils. The sense relay coils are driven by the 18V brick outputs. The control relay coils could be any voltage, even DC. It would be possible to avoid the extra power supply by using one of the bricks.
Some time ago I sketched out a design to enable track power only if the TMCC signal is present. The command base was powered first. After a delay a relay turned on 120V to the bricks and transformers if the command signal was present. Then if the command signal dropped out the power was shut down. It used an Amperite time delay relay tube - I inherited a bunch of these in various voltages and delay times from a friend.
CJACK - Detecting a derailment that doesn't trip the breaker is a challenge. How to differentiate this from a normal current load. It might have to be a dynamic check looking for a sudden change in current? Maybe use a video camera with derailment recognition software. 
