What setting of the reverser may cause the inability to drive a two-carriage tram?

The reverser is responsible for setting the direction of travel of the motors in each car. In a two-car set, where the cars are facing in opposite directions, setting both reversers to the 'P' (forward) position will cause each of them to attempt to travel in its own 'forward' direction. As a result, both cars will pull in opposite directions, which will prevent movement of the entire set.