Ross wrote to uk.transport.london on Fri, 24 Sep 2004:

And to further complicate matters, retention tanks require that the
contents be emptied, which means that they need somewhere with a
suitable cleaning pan and that in turn means that it's unlikely that
tanks can be cleared at every poxy little stabling point at which
trains get left overnight. In the case of the 170, the toilets will
lock themselves out once the retention tanks are full.

I think it is the same everywhere. Although, to be fair, Eurostars seem
to manage to have loos that flush for the whole journey and so,
normally, does the Shuttle. But there, I suspect, if one loo locks
itself out of service, the others don't, unlike on surface-only trains.
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 11 September 2004