Halliday & Matthiessen (2014: 558):
Note that we have to differentiate group and phrase complexes from clause complexes involving ellipsis. In clause complexes where the Subject or the Subject and the Finite have been ellipsed in a continuing clause, it is easy to see that a whole clause is involved:||| Then he went in the Navy || and [∅: he] helped design various gunnery training devices [[ used during World War II]] . |||
But the whole clause is still involved in cases where other elements have been ellipsed:The Land-Rover was to take him to Santander, then the train [∅: was to take him] to Bilbao for the late afternoon flight.
Here the train to Bilbao for the late afternoon flight is an elliptical clause consisting of three explicit elements – Subject: the train + Adjunct: to Bilbao + Adjunct: for the late afternoon flight, with Finite/Predicator and Complement left out by ellipsis. The general principle is that as long as only one element is involved, we can analyse the complexing at group/phrase rank, but as soon as more than one element is involved, we have to analyse the complexing at clause rank and posit ellipsis in one of the clauses.