Originally Posted by
cbn42
Direct flight means same flight number, not necessarily same plane. Subtle difference, but can be important.
On Southwest it generally does mean same plane, or at least they were planning to use the same plane. WN typically only subs another aircraft in the way that the OP describes if the original plane is running behind schedule.
On the other hand, legacies run scheduled "same flight number different plane" direct flights all the time which is something that I personally think is a bit of a shady practice.