Hi - I don't visit cinematography.com very often now, but just happened on this question.
David Mullen - as always - remembers well. In fact it was often common to make an IP before the answer print was finalised - or immediately afterwards. That IP would become the master from which Dupe Negatives were made for the release print run. But also a the IP would be used to print a number of short sections (on an optical step printer) that included all the shots for the trailer. This might be frame accurate, yielding a one-piece trailer negative, or possibly, if full takes were printed to minimise handling damage to the IP, then the dupe neg would have to be fine-cut.
However, since as many trailer copies would be required as prints of the feature itself, it would not be unusual for the trailer to go through two more generations to produce additional dupe negatives for bulk printing.
In earlier times it was common to make the trailer from out-takes, so it would follow exactly the same work-flow as the feature: edit work print, match-cut the original negative, then make an IP and an DN. However, later there was a requirement that the trailer only included material that was in the actual feature (to avoid accusations of false representation). Now it is quite common to shoot and release trailers during (or even before) production on the main picture but the questions of generations of film, and of exact shots, are either meaningless or irrelevant!
Finally, I can't be certain, but I am pretty sure that Titanic's trailers would have been made in a moe traditional way - but yes, film-outs from a digital master using an EDL and film recorder would have come in soon after that.