Yea, Hawaii has had several dozen railroads over the years or some such IIRC.  Alot of the Sugar Plantations basically had their own for a long time, and at one point alot of those were unified as one company for awhile until the industry lost its sway.  I've seen some pretty wild pictures of some of the little narrow gauge locomotives they used to run....  OSHA would scream and cry if they existed today....
@The OP
I can tell you've spent quite a bit of time working on it, and its to your credit.  Few people get a route that far along.  I'd ask, how much more time do you want to spend on it though?
Recommendations would start with Track work and Catenaries.  If the train goes around a corner and the Diaphragms between the cars split so hard you can see the end of the rail cars inside the diaphragm, the corner's are too tight (Which they definitely are,  Id suggest twice the Radius at a minimum on some of those curves.  The one at ~7:10 or so in the film had me cringing waiting for the sparks and derail, screaming people and sirens....).  
I would start with the Track Straightener Tool.  Decide which pieces of Track are meant to go between Point A and B (Spline Points), and be entirely Straight.  Straighten those....  That will clean up alot of some more "wavy" lengths of track you've got.  When you have a Forced Spline point (Track Change, such as a tunnel entrance, or Elevation Change Spline point), you need to decide if you want the track to keep going straight.  If you do, straigten both sides and align as best as possible.  If not, pick one side or the other and straighten.  Ideally you want as few pieces of "Straightened Track" next to each other as possible.  So "S,C,S,C,S,C,S,C" etc etc (S = Straightened, C = Curvy).  Doing this will also help with multi Track Mainlines (Particularly since you seem to have alot of 3-4 track mainline), because then you can just line the spline points up and they'll align REALLY easily.
Your switches are a little disjointed too.  I can tell because when your train goes through them, it jolts to one side or the other, and then the cars behind it follow it and repeat the same Jolt.  Basically you need a more gradual Radius in the Turnout to eliminate that.  The easy method is to Straighten the Track that Forms the STALK of the "Y" created by a Turnout (But don't straigten the Arms if they have any radius to them at all, IE if they branch away from Straight on in relation to the Stalk), then ease the Arm or Arms that branch away the hardest through a gradual corner, rather then having a hard angle or a high degree radius.  If all of that makes sense.  It takes some practice to do switches well, and when you throw goofy landscape/terrain issues into it can be annoying sometimes, but nothing will ruin a good train sim for someone then watching their Train do the Konga through a series of Switches.
Catenaries:
My suggestion (Entirely up to you), is to turn off the Auto-provided Catenaries and pick a lower Catenary height that matches the trains you're using.  You're already placing Catenary poles anyway, it won't be too difficult to place some with a spline Electrical line at the correct height.  See the various works of an author named "TUME" for some guidance in this regard if you choose to pursue it.
Scenery:
After that, the only scenery counsel I would give would be to find some different textures just to break things up.  I like the textures you've used, but there seem to be alot of the same.  I would also point out that the color makes this seem like a denuded Volcanic Island or something.  Not sure what your intentions here were, but thats what it seems to me at a glance.  If you broke that color up more, the scenary would become more interesting or appealing to look at (Not that you haven't done a good job of that already, just to enhance what you already have).  The textures you use can also play into how you want the Island to feel.  Do you want Volcanic Reclamation?  Or Tropical Paradise?  Maybe Splinter of Britain?  Where you go is of course up to you, but this kind of thing can greatly feed into what you're trying to accomplish, whatever that might be.
After that keep up the good work.  You're doing well.  The above is meant only as suggestion for what this user would think of as improvements.
Good Luck,
Falcus