I was just thinking about something like this.

I'm not sure you'd actually need to have physical coupling from the train to the platform. Train lavatories used to just dump their contents out of the track. Given that tube trains are already required to stop at a very precise location on the platform, perhaps the hot water could just be dumped into a funnel built into the ground underneath the track beneath the rails.

Getting cold water onto the train would be harder. Perhaps a big funnel on the top of the train underneath a pipe? ;)