A few references I found on Google which may help.
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Great Eastern Train Ferries Ltd. Opening of Harwich - Zeebrugge Train Ferry Service Thursday 24 April 1924. Inaugural Luncheon at the Felix Hotel, Felixstowe.
Die Reichsbahn
16/1925, Fährbootverkehr Deutschland - England über Zeebrügge - Harwich
1925.
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Also found some more sources and info
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details: RAIL 390/1232 : quick reference Purchase and re-conditioning of SS "Essex Ferry" formerly "Princess Iris" formerly Train Ferry No.1
1946-1947
Covering dates 1946-1947 Scope and content Purchase and re-conditioning of SS "Essex Ferry" formerly "Princess Iris" formerly Train Ferry No.1 Access conditions Normal Closure 30 Held by The National Archives, Kew
Restrictions on use Closure status Open Document, Open Description
Two different posters
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Harwich Train Ferry Traffic Comments: See 'British Railways Illustrated' Vol 2,1992/3, page 284-286 'LNER's Entry into Europe', includes consideration of traffic before and after WW2.
Picture of Train Ferry at sea
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also The Harwich-Zeebrugge Train Ferry, Leaving Harwich
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Zeebrugge Photo
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A Belgian train ferry wagon on the L.N.E.R.http://mikes.railhistory.railfan.net/ph046/04230.jpghttp://mikes.railhistory.railfan.net/r046.html