The rain is not in the screenshot because of the way the particle systems are handled in the render target texture. See this thread for more info: Screenshot and ParticleSystem - #2 by Evgeni_Popov
To sum up: if a particle system has no emitter or has an emitter which is not an AbstractMesh, the particle system is not rendered in the texture.
In fact, the emitter does not have to be an AbstractMesh but it mush have a position property and a isEnabled() method. Also, this emitter must be in the list of the visible meshes of the frame.
So, you have a way to make the rain visible in the screenshot: set a dummy value to the emitter property to all the particle systems before taking the screenshot and reset back this property after: