Subject: How do fluorescent neon tube lights produce a striped zebra effect?

I'm asking this question for a friend of mine who has been baffled for ages now
as to how the fluorescent strip tube lighting you find in underground trains etc
sometimes produce a striped effect that may appears to move along the light, one
way and then back again. He is under the impression that electricity enters the
light from one end and passes through, exciting the neon gas which reacts with a
powder to produce the visable light. How is it possible for the tube light to
produce a striped dark and light effect (that appears to move) and not just be
fully on or off if filled with a gas? 

Re: How do fluorescent neon tube lights produce a striped zebra effect?