Why are there 4 wires leading to the Power Functions LEGO train motor?
The LEGO Power Functions connectors have 4 standard designations regardless of the usage:


9V power
"0V" Ground
C1 "control 1"
C2 "control 2"


It seems for motors that the 9V and Ground are unused.  The C1 and C2 are used in combination to pick the motor direction.  If C1 is providing power and C2 is providing ground then the motor goes forward.  If C2 is providing power and C1 is ground then it goes backwards.



The dashed lines are unused at that stage in the flow.