It is fast and easy to make "real-looking" rain in Combustion by using the built-in particle generator.....
First, create a project with a transparent layer of standard rain particles.
Next, to the entire layer, apply an unconstrained vertical blur.
Lastly, to give some excellent rain-depth feeling, COPY and PASTE the layer to multiple times. You create depth field feeling by simply changing the blur amount on each layer:
close up rain, medium rain, far away rain etc. To do this simply alter the blur value on each appropriate layer so it fits with whatever your shot, based on your focal point.