In full "Rheingold Express". A luxury train which ran along the course of the Rhine between the Hook of Holland (near Rotterdam) and Basel.
1920s; earliest use found in The Times. From German Rheingold, lit. ‘gold of the Rhine’ (from Rhein, the name of the River Rhine + Gold), in Das Rheingold, the name of an opera by Richard Wagner (first performed in 1869), the first part of the tetralogy Der Ring des Nibelungen.