I've been studying Philo's page, particularly the NXC code and it looks like he is using
NXT port A for the 9v and NXT port B for the control. I can imagine what the cable may
look like, but didn't see a diagram on his web pages. I did post a question on his page,
asking if he could post a diagram of the cable he used. I also proved to myself that the
LEGO adapter cables don't work, but don't harm the motor. It still does work, and
quite well with the train RC 8879. Controlling the 9398 with the 8879, allows good
speed and steering control.
But having to use 2 motor ports to control the servo motor negates its pluses. Maybe the
next Mindstorms will interface nicer to PF.