That's because the retention tank was designed as if the train wouldn't be moving... hence why the aircon inlet is right next to it...
Well, it doesn't move on paper.
I'm surprised thats not a Health and Safety issue.
As I've said on another post I understand there is a design fault, the waste pipe is ribbed to with stand the external pressure when vacuum operates, the pipe bends are not so leakage occurs if the filler in the bends fails.