As I'm sure most of us are aware the discharging of toilets directly onto the track is now greatly frowned upon and we are slowly moving towards all trains having retention tanks fitted. A question that has occurred to a few people I was with today however concerns greywater (i.e. water from sinks or showers rather than toilets). The new Caledonian Sleeper stock will have showers on board and we were wondering if they will be allowed to dump this greywater onto the track or if it will also have to be stored for later disposal/treatment? If it also has to be stored then presumably the Mk5s will have to have very large water tanks for the showers and very large storage tanks for the greywater from the showers!