In order to eliminate this, we have to go over a few steps covered in the previous guide. If you are already familiar with the basics from the original guide, you can of course scroll down.
The tool resides in the middle left hand menu, under "misc" and called "Tunnel Occlusion Volume"
In order to now use the tool and fix the unwanted moisture, we first have to place the tool. Fig 3 shows the tool in its placed, unselected state.
As you can see from Fig 5, I have once again pre-sized the tool in order to fit the structure.
The result is as can be seen below (or cannot be seen clearly, you will have to trust me on it) whereby the inside of the structure now has no rain.
This tool really does have some fantastic implications, not just on the tunnels and major structures such as stations and sheds, but can also be used more or less anywhere it would be required, under bridges for instance.