Ferry wagons were probably not as rare or obscure as we may think.
I suspect it's more than likely many were not seen during the day as they were always being moved at night to coincide with the Harwich cross channel rail ferry sailings. Harwich then being the major rail freight cross channel port. Dover was also used but it was more famous the 'Night Ferry'.
For more information about operations I recommend watching Link Span, the British Transport Film produced in 1956 which shows the Harwich and Dover rail ferries.
Good model
, hopefully it may be complimented with the DB and FS versions .