I have a Chessie SD40-2 with a rear coupler that tends to uncouple randomly. The track is flat (GarGraves on ultra-low pile carpet), the couplers line up just fine when coupled. It doesn't seem to make a difference whether the coupled car is heavy or light.
Is it wise to attempt to disassemble the rear coupler assembly and try and stretch the spring a bit to tighten it up?