Pretty Impressive train SD90.

Im a member of NTrak and long trains are common among our community, however, we tend to run them long in order to see how many cars a particular locomotive will pull or how many locomotives it takes to pull everyone's cars.

We do take some photos of these trains, however, the purpose is to show only the length of the train, as any detail is lost due to the distance of the camera from the train.
Most big layout owners (in my experiance anyways) don't run extremely long trains and those that do, don't have the space to be able to show the entire train in the open unless they are modeling the great plains,
As an example, Featured in the July-August issue of N Scale Railroading is an article on Brian Morgan's Selkirk Division.
Having had the honor of operating on his layout,I have run train #57 which is a unit grain train and, depending on the session, can run anywhere from 50 to 90 cars in length.
Even though Brian has a large layout, there is no way that the entire train can be seen at once as the tunnels, rock cuts and other scenery tend to block it out.
I have done a lot of railfanning over the years here in Western Canada and I have never been able to get an entire large train in one shot and,If you are modeling a mountain scene as I suspect, it will cost you a lot of valuable real estate in order to model a scene which will allow you to get an entire long train in one shot.
Please don't get me wrong, as I am not criticizing your ideas, but I do feel that your long trains will look Grand snaking their way through the tunnels, rock cuts, over the bridges and through the scenery of your layout and that they will photograph very well even if you can't see the entire train at once.
Have a great one and Happy modeling!
Cheers!
Terry