This is just a shot in the dark. I had an issue years ago with a simple lift out bridge. It was wired to the main layout and isolated from the yard. The main layout was on a different channel than the yard. With a change in weather the benchwork and everything shifted a bit. The 2 channels joined one another through the center rail sending the track signal south. While juggling some cars on the layout before running. I blew a fuse in the yard. Without realizing it.. With the center rails touching now. One channel was trying to feed to much track. The permanent fix was to trim the track and epoxy a piece of styrene between the 2 center rails.
The fact that you have a lift bridge. Any chance you may be bridging 2 isolated blocks with the rails slightly touching. The fact that the engine on it's own generates a 10. It seems the cars are the issue. Can you park the engine in the block with the track signal running. Should be a 10. Using your hand push one passenger car into the block at a time. Just to see where the change is in signal. As far as your wiring. Using 12 gauge wire that should be fine for a long run. Is the 12 gauge a buss wire ? Or does it go directly to the rails as a feeder ? Remember. Even though you have isolated blocks. The passenger cars will bridge 2 blocks with their rollers.
