I am very keen to learn more re DC + DCC wiring. Read all the 'don't do it' articles. Glad to find someone who has done it and had no problems. Wonderful!
I simply have too many DC engines which are not suitable for conversion and I don't want to part with them. Thought about tracks of DC only and DCC only. But with about 100+ engines, I'd like flexibility of using any track for either.
The double pole, double throw switches way seemed too risky. Tried to find what you have and could not. Instead, I've decided to wire each of my my circuits / tracks to a 25 pin socket. Then two different plugs - one for the DC system and one for the DCC system. Only wiring and no systems nor track power dependent accessories between track and socket.
Admittedly there is still the human factor - matching engine to system. But the effort of changing plugs should reduce the risk far lower than simply / accidentally flicking a switch.