A few things for a believable video!
For some people, train sims are a way to express a certain kind of creativity and passion they have. It could be driving, making routes, taking pictures, or making videos. For me, I like to do all 4, but in this, we're going to talk about making believable videos. There's a lot more than smoothness that goes into making a believable video.
Operational Fundamentals
Prototypical operations is a critical part of making believable videos of any kind regardless of what era you're doing. For a lot of us, its taking a trip down to the rail-yard, or mainline closest to you, and watching trains go by. Its amazing what you can learn from simply watching what the real guys do everyday. Youtube is also a very good helper in researching because it also contains material regarding prototypical operations of different era's. For the extreme railfan, you might have some DVD's and VHS tapes showing some oldies but goodies stuff. All of these are good sources. It takes hours of simply watching to see what goes into the day-by-day operations of a real railroad.
Something that always hurts the believability in videos from any train sim, is the continuous running of trains at run-8. In real life, trains do not run at run-8 24/7/365. It's always a good rule of thumb to check track speed limits, grades, and signals before starting the train for its journey past your virtual cameras. I like to set my trains up a longer distance from the virtual camera's so the trains don't always have to run past the cameras at full power, thus giving more realism to the video. If the route you're using however, has a large grade on it, then it is more acceptable for trains to be running at full power for an extended period of time.
Another thing to look out for is speed. Not all trains will be traveling 70 MPH. Its always a good rule of thumb to check speed limits BEFORE starting the train. When speed limits are known ahead of time, you can adequately drive the train at a realistic speed setting for the video, thus making it more believable for the audience. I usually like to keep my trains 2-5 MPH below the posted limit, because on UP, this is how trains travel a lot of times.
Something that always hurts the believability in videos from any train sim, is the continuous running of trains at run-8. In real life, trains do not run at run-8 24/7/365. It's always a good rule of thumb to check track speed limits, grades, and signals before starting the train for its journey past your virtual cameras. I like to set my trains up a longer distance from the virtual camera's so the trains don't always have to run past the cameras at full power, thus giving more realism to the video. If the route you're using however, has a large grade on it, then it is more acceptable for trains to be running at full power for an extended period of time.
Another thing to look out for is speed. Not all trains will be traveling 70 MPH. Its always a good rule of thumb to check speed limits BEFORE starting the train. When speed limits are known ahead of time, you can adequately drive the train at a realistic speed setting for the video, thus making it more believable for the audience. I usually like to keep my trains 2-5 MPH below the posted limit, because on UP, this is how trains travel a lot of times.
Locomotive choice
This aspect of believable videos is something that is often seen as not important but should be treated with respect. You're choice of motive power to use should coincide with the type of railroad or era you're videoing. If you're doing the 1950's, you wouldn't exactly expect to see a GE ES44AC or EMD SD70ACe locomotive leading a train in that time frame now you would? Road names should also coincide with the particular time period or area you're videoing. There can be foreign visitors, but like in real life, you shouldn't expect to see them all the time. A CSX locomotive wouldn't very well look good on a 1960's SCL train right? With the modern era its a bit different, there's a large number of locomotive models that might get seen today. Still beware of what choices you make, you can still turn people away by taking non-prototypical power for the modern era trying to make it prototypical. It just won't work.
Sounds
Realistic sounds are nearly as important as prototypical operations. Every good believable video needs good realistic sounds. Sounds refer to enginesounds, hornsounds, and railcar axle sounds. Some locomotives for train sims have incorrect enginesounds for the locomotive type. This is no big deal as they can be changed to a more accurate one. Its a good rule of thumb to listen to the engine sounds and hornsounds before videoing. This will give you the chance to decide if you are going to use them or not, and if they are accurate for what you're doing. Realistic sounds convey that special message that real life trains do as they roll past you.
Train Types
Its always good to use realistic train types based on the railroad and time period you're modelling. Again, research will be needed in determining what types of trains you could place in order to have believability in your production. Not all railroads are alike, just as so not all trains are alike. Since this is the case, you cannot simply say, welp, CSX does their trains this way, and run these types of trains, and then assume that NS, CP, CN, BNSF, and UP do that as well. It doesn't work like that. Research is critical in the placing of accurate trains and the power commonly seen on them. There are so many good sources to find this information. This accuracy can sometimes take days to make happen before finally you get to video that one train. You have to be as patient as possible when working with this. Trust me, you are going to get frustrated when you find out that some of the things needed are not available. Some compromises can be made in this, especially when working with mixed freights. You can work around the non-availability of a certain asset to still achieve a believable effect.
AI vs Playing it out by hand
While I know this is just my personal opinion, I believe that a more realistic effect can be achieved by doing it all out by hand. Yes it takes longer, but you are in control of everything. Trainz AI is known to be quite stupid, and stop places they aren't supposed to, go down tracks they were never meant to be on and etc. When you're the engineer, this doesn't happen. How you do it is up to you, but I prefer to do it all out by hand. You'll also find you have much more fun doing it out by hand, because while you feel like that film guy you've always dreamed of being, you also feel like that engineer as well.
After filming editing AKA "photo-shopping"
This is another one that is up to you really. While this can help add extra impression on particular era or something of the sorts, it can also hinder the believability in certain aspects. Being to experimental can ruin a production in a hurry, so its best to kinda just start slow. Fading in from black effects at the beginning of the video like you would see on DVD, transitions between the multiple videos that are combined together, and fading out to the black effects are what I most commonly use in editing the big video after being done. Black-and-white effects can be added for videos that are supposed to portray an era back in time before color filming. There's a lot of effects that could make a video better, but as stated before, you can go to far. It's best to really just start small, see what you can do. Always, and I mean ALWAYS watch your production before publishing. You can catch the errors before the big debut that will hurt this video big time and fix them.
Good Sources for research
These are a few of the sources I use for researching stuff when I am looking to get serious. I'm sure that they can help you too!
http://rrpicturearchives.net/
https://www.youtube.com/
http://trc.trains.com/ (Trains Magazine)
http://www.railpictures.net/
https://www.yahoo.com/ (Yes a search engine. Those always come in handy!)
https://www.google.com/
And of course another good place to check is your friends. They might have some good stuff to share with you as well. You'd be amazed what you could learn from them.
http://rrpicturearchives.net/
https://www.youtube.com/
http://trc.trains.com/ (Trains Magazine)
http://www.railpictures.net/
https://www.yahoo.com/ (Yes a search engine. Those always come in handy!)
https://www.google.com/
And of course another good place to check is your friends. They might have some good stuff to share with you as well. You'd be amazed what you could learn from them.