Traditionally, straight DC 2 rail pickup is: right side rail from the engine, left side rail from the tender. Therefore there is one wire from the tender to the engine.
Check your wheelsets for continuity (insulated or not) on the engine and tender. Your result should confirm the explanation.
For example: the engine wheels and frame may be electrically 'hot' throughout, except for the insulated left side wheels. Insulated right side for the tender.
A second alternate example would be power is picked up through iINSULATED wheels: right for the engine, left for the tender. The presence of wheel wipers on the insulated wheels would show this.