Series Wound DC motor Runaway when going downhill
In your scenario, there is a load.  The motor is connected to the drive train, so there is always some kind of load.  It is doubtful that anything would cause the motor itself to be in danger of damage.  Running into a tree or something else at the bottom of the hill will certainly damage the cart, and probably the driver, but doubtfully the motor.

Pure series wound motors do suffer from potentially destructive speeds with no load on the output shaft, but that is due to the fact that with no load, as the motor accelerates, the current through the windings, both armature and field will drop.  As the field current drops, so does the field magnetic strength, which could possibly allow the motor to accelerate to dangerous speeds.  

The advantage of series wound motors is that when there is a very high load, the field strength increases, which allows the motor to generate more torque, which is necessary on traction motors to get things going.