Q20.104. Severn Tunnel Motorail

Hi, I am trying to find information on the motorail service that ran thru the Severn Tunnel prior to the opening of the first bridge in 1966.  I believe it ran from Severn Tunnel Jn to Pilning.  Is there information in the archives about this, or other publications elsewhere.  I am particularly curious to know if it was ever diagrammed for diesel haulage. And if so what.  (Robert Marshall)