Doug, its not just over kill but it could be dangerous to your trains. If you derail between 2 power districts powered by two different bricks you've got 20 amps of power with next to no circuit protection (the breakers on the bricks wont trip reliably if at all sometimes if you've got two in parallel). Ive smoked out a brand new legacy loco due to this.
Matt,
I may be misunderstanding you comment, but I am not sure what the problems is. Are you saying if an engine with two pickup rollers derails, with one of the pickup rollers on each side of the insulating pin between power districts, that the engine will get 20 amps to it? This would be an issue wherever you cross between one block powered by transformer A and another powered by transformer B, and I have not heard of problems before.
There have been many discussions on the benefits of power districts a long mainlines (I personally don't do that), and those that recommend this have not stated this as a problem. If it really is a problem, how do those that use power districts solve the problem?
On the question of overkill, you can never have too much power
Ron, if I'm not mistaken, I'm the one who advanced that concept to Matt. I had a pretty good disaster in which I fried all the wiring in an engine. It was a perfect storm scenario.
I use 6 amp polyfuses for my over current protection. What I only recently learned was, they are very slow acting. They take 10 seconds to kick in and cut the power. That's too long, and I am going to have to find something faster.
Back to the derailment: the engine split a switch and came to rest with one truck in one power district and the other in a second district. I use 20 amp power supplies. Both districts are fed off the same supply. I don't know how many amps went through those wires, my guess is a minimum of 12, but it may have been the whole 20, because of the slow reaction time of the polyfuses. The wires inside the engine were probably 18 gauge and can only handle about 2.5 amps. The smoke was horrific! If I had had a fire extinguisher handy back then I might have reached for it. No desire to relive that and the extinguishers are in the house.
In a separate, more recent incident, the derailment was so bad that it took out the fuse on the primary (wall) side of the power supply. Amazingly, the engine escaped unscathed, but that only reinforced the deficiency of the polyfuse for this application. It literally "doesn't cut it".
The bottom line is, if you aren't very careful, it is possible to get unexpected and / or undesired results.