You must first take a ferry across the Channel, as motorail trains do not use the Channel Tunnel. They now run from Düsseldorf in Germany to Verona in Italy. The best information resource is The Man in Seat 61.
N.B. There is no motorail service in France at the time of writing (March 2020), as the French Railways motorail service Auto-Train from Paris to Nice was discontinued in 2019. (The service down through Italy stopped many years ago).
For general rail enquiries:
|Voyages sncf (formerly Rail Europe) http://uk.voyages-sncf.com/en/
The Man in Seat61 www.seat61.com/Motorail.htm
The latter is a veritable mine of information on train travel generally and comes highly recommended.
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