John, I do recall now that you mention it that the TIU has TVS. How quickly I forget. What you did makes sense to me, now that I think this all through. That dovetails into my response to Michael.
Michael, I am happy we can have this discussion here. I really do not know about whether you can have too many TVS's or put protective devices in too many places. My background in electronics is mostly with technologies that are buried in landfills or have been melted down for other uses. 
Most recently (the last 20 years) in telecommunications we had fuses for the power feeding the main office equipment, surge protectors on the cable going out to the next location, protectors on the opposite end of the cable, and fuses in the remote end itself. Translate that to our layout, you have a breaker on the AC circuit feeding the Z4000, an overload in the Z4000, the faster responding PSX between the Z4000 and the TIU to stop power to the TIU, a TVS on each channel in the TIU, so it would seem a TVS at the track end of the wiring of each feed would make sense to me. What would re recommended by those in the O gauge train industry probably varies as opinions varied in telecom and the electric power industry.
There is probably something on this Forum that discusses these questions further. I would just need to do some searching.
Thank you again everyone who has contributed with questions, information, or just browsed and didn't leave comment. I'm sure it got many in the later category thinking as many topics here get me thinking. Smell the smoke?? 
