from that last issue sounds like both locomotives think the motors are stalled check and make sure there is nothing in the gears.
As for the couplers that’s odd. There is a little PCB on top of the truck that if it gets turned too tight it will unplug. One of mine had this happen you can check there and make sure they are plugged in but for both of them to not work is odd. You have 18VAC on the track?