Draw line by motion guide?

Hi i have a problem. I need to make something like a comet moving by a complicated track.
Actually just a line moving by a track slowly dissapearing on earlier stages of the track. I thought it will be possible to do it with usage of motion guide, just moving a point on he track with something like motion blur falowing it. The directions change constantly.
Anyone could help with any ideas how to start with it?:slight_smile:
Actually i want to get something like this effect,
http://www.flashkit.com/movies/Animations/Scratchpad/Effects/MovieCli-Pierre_S-7113/index.php
but the trace have to be longer, and the object have to be continuos line, not balls.