Quite possibly the biggest bitch using the X-Ray 1.0 engine, creating a natural, realistic rain texture that also looks like its being dynamically generated by the engine itself (think like the rain in Metro 2034 on outside levels). Its not easy, to say the least, given the X-Ray 1.0 limitations and general primitive nature of it but the following guide will ensure that with a little practice as randomly placing some white lines you get the best possible rain texture.
A note on SD and HD canvas sizes: Personally, I prefer the SD size as it seems to get arbitrarily stretched/increased to a certain size in X-Ray which leads to the rain getting slightly blurred leading to a more natural look. Of course, this may simply be entirely down to the random placement of lines I created simply looking better this way, your own result might end up looking better when made in HD. I'll upload this guide and a few working examples to help you along in the downloads section.
To make a rain texture with paint.net for SoC:
1. Create a canvas of 56x28 (HD) or 20x10 (SD)
2. Fill with solid black
3. Randomly mask off varying lengths of 2 pixels (HD) or 1 pixel (SD) wide lines with rectangle select using Add (union), then fill with white.
4. Select magic wand using Replace, select the black background
5. Copy the black background, then Cut it
6. Select Layers > Layer Properties, start by using a opacity value of 5 or 6
7. Select Layers > Add New Layer, paste in the black background you copied
8. Select Layers > Layer Properties, set opacity to 2
9. Select Image > Flatten
10. Save as fx_rain, DXT5, Iterative Fit, Perceptual, Weight Colour By Alpha, no Mipmaps
Now Create another canvas of 384x384, fill it with a grey, its probably best not to use too dark of a grey colour.
1. Select Layers > Layer Properties, set opacity to 5 or 6
2. Select Image > Flatten
3. Save as rain, DXT5, Iterative Fit, Perceptual, Weight Colour By Alpha, no Mipmaps
Now just place the files in textures > fx, next time it rains you will see the fruit of your labor.
You can see a video of the rain texture I created here; Moddb.com