See my other post. This appears to be a shunt field motor with a compensation winding, based upon your description and nameplate diagram.
It is possible that this motor will behave like a series motor if the shunt field is disconnected. In fact, this makes sense with respect to your initial description of its behavior. Series motors don't have a distinct maximum RPM as shunt field motors do. Under light load, the motor can continue to develop a small amount of torque in the forward direction where shunt field motors have a distinct point at which they cease to provide mechanical power out and, if driven from an external prime mover (or the inertia of the vehicle) will become generators, providing dynamic braking.