Okay so I've been playing around with layers in Photoshop again. I thought it might be a nice idea to share my technique in case any other artists find it helpful. After building the scene, I did an initial render pass of the model and the train carriage in Poser, followed by a second render with the just the model. I opened the first render in PS and dropped the background in behind it. I then added a layer between them painted with a brush from Ron's Fog. I then added another fog layer in front of it (with the fog painted in slightly lower). I then dropped the second render on top. One last layer with the rain (Ron's Rain, naturally) on top. I set that layer's blending options to "screen." Finally I flattened the image and applied a soft focus filter.