As far as I know with the current state of textures/seasons available, you will need two shape files, one for normal and the other for rain. Textures are called by the S file and normally reside in the same folder as the shape ( S ) file. You need to make the rainy textures, name them a bit different to the originals, then open the S file and change the tile calls to use your new rainy textures.
Always make a copy of the loco/wagon you are working on, so if things go wrong, you still have the original.
Beer is not a matter of life or death, it is much more serious than that.