I always get a kick out of these discussions; they make us aficianados struggle to wrap our heads around how "alien" people (movie makers) think...
I'm not in the movie industry, but I do know enough about CGI to assert that Really Good CGI is still pretty expensive. There are folks who post here who are working on realistic CGI representations of locos and rolling stock we admire; ask them about the effort required to build these models.
In general, after spending a lot of time scratching my head about the motivations behind the train scenes I've seen in movies, I think the two influences driving these guys are 1) the story to be told, and 2) how they want you to feel about that story. Realism may be a factor in either or both, but it's not the reason, unless you're doing documenaries (and IMHO that category is also suspect, sometimes...)
Finally, the mercenary and prostitute in me is mostly forgiving of some of these "decorative travesties" inflicted upon our ferroequinal objects of affection, as long as they pay good money and don't make things too hard to undo.
(Note: the above was written under the influence of probably one too many cups of coffee...)