The connector lead used to connect the battery box to the motor unit. Two 2x2 black plates joined by 9cm of wire. Similar to other 9V connector leads used in Train and Technic but originally only 1 flat high.
Recently, this part seems to have been replaced with a 2 flat high version with the same part number. This reduces the clearance between the motor and the cabin when the train "bends" to go up a slope but doesn't seem to be a problem. The 2 flat high one is shown above.