Troy Steadman wrote to uk.transport.london on Sat, 18 Dec 2004:

Didn't there used to be trains that instead of disgorging their
passengers at the docks actually drove (drove?) steamed on to sidings
on the decks of ships then steamed off Stena-like to continue their
journey across Europe?

There was indeed - the "Golden Arrow" ran from London to Paris
overnight, via Dunkerque. Only first-class passengers (I think I am
right in saying) remained on the trains; the others had to get off.
This was the "Night Ferry" service beloved of generations of students,
and used by many of my contemporaries as a cheap way of travelling
between the two capitals. But the train service was considered the last
word in luxury when it was inaugurated.
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 18 December 2004