the 0 volt connection is via the rail, to be more precise the rack and pinion gear, make sure the rack is clean, no build up of compacted fluff and grease etc in the teeth, normally a brisk going over with a wire brush to remove the build up is enough to cure the issue, then a little grease to relube it, if its not then you may need to clean the pinion gear, which is a carriage off the rail job to do it safely!
also try taking a scouring pad to the copper strip to clean it up, removing any tarnishing so the charge pin gets a better contact, and check the condition, length of the pin and replace it if needed
