I don't know if this is what you mean by a "diagram" but see if this helps.
To be clear this uses the method of switching the "hot" brick power. It is assumed that all the brick commons and track block outer-rails are tied together.
I found the following items available now on eBay though these have the 2-3 week delay from Asia. I'm sure all these items are available domestically (Amazon, eBay U.S. sellers) though my experience is this double or triples the cost. As shown it looks like between $10-15 not including wire.
The electronics operates on 12V DC supplied by a wall-wart so you obviously need an available wall outlet. IF this is a nuisance, an option is a ~$3 AC-to-DC converter module that would convert 18V AC from one of the AC bricks into 12V DC. For example:
Everyone has there favorite way to assemble electronic circuits so to each his own. One way is to use so-called prototyping board which has solder-able holes on a 0.1" (2.54mm) grid. I show below the use of an IC socket for the optocouplers. This way you solder the interconnection wires to the socket and not to the IC terminals. The current flowing within the board is very small so something like #30 gauge solid would work fine. It's also negligible current flowing between the bricks and this board, and this board to the relay module so thin wire can be used too. Practically speaking these interconnections would use, say, #24 or #26 gauge or whatever you have lying around for hook-up wire.
I suggest using 2-pin screw terminal blocks on the board: (4) on the input side from the 4 AC bricks, (1) to provide DC+ and DC-, and (1) for the trigger signal to the relay module.
I also don't show the use of terminal blocks, wire nuts, or whatever hookup method to distribute brick power as that's dependent on your layout and what you use today.
If you're using Clem's system as a model, I see he has some indicator/status lights and toggle switches to bypass/over-ride the safety shutdown function. We can explore exactly how those would integrate or if you even need/want them. First though, I suggest contemplating the above to re-affirm your decision to jump in.
But to your point, if your son is going into Controls Engineering, he and optocouplers will soon become BFFs. 
