bajinnova posted:...
I ordered one of the buck converters and will check to see whether I can use a common common between the AC and DC sides, if not I will try the wall wart. I guess I should be able to figure out if the common commons are possible but easier just to test it.
Well, I suppose it might be easier to just test it but I assure you it will be a sacrificial buck module albeit "only" a $1 investment. Above diagram from a previous OGR post discussing the very issue of mixing DC common with AC common. As shown, point A (the AC Ground/ common) is NOT the same as point B (DC common). If you short A and B you will effectively be placing full AC voltage across one of the diodes inside the bridge rectifier. This will blow up that diode - game over! 
Above is yet another diagram from a previous OGR thread. I think this kind of represents what you are trying to do with blocks - ignore the "MTH signal" on the right side. The wall-wart provides electrical isolation (via transformer windings) so that its DC- output is isolated from the AC Grounds/commons from the train transformers which of course have their own isolation windings. It is this isolation between the train transformers and the wall-wart that allows you to then tie together the wall-wart DC- output and the train transformer(s) AC common output(s).
