Interesting looking route you got there.
I totally understand where you're coming from with the issues you're having about the Sidings and the Scenery. Takes awhile to figure out what you want to do if you're not Mr Train Wizard (lol).
I could bring up lots of things to discuss, but at the end of the day, its really what you want to do. Have any goals for this route? Certain level of prototype you're shooting for? or not? Answer those for yourself and the answer's to most of the rest of your questions as far as the route is concerned will get alot easier. At the end of the day this entire hobby is about what you want to do. If you want Prototype, you have to create reasons for why the Railroad laid this spur here or that siding there. If you just want easy, or interesting operations, lay track where ever it appeals to you to have it. Unit Trains make operation easy, but they're rare outside of some few narrow industries or certain parts of the nation, and if you're going for Steam Transition Era, they'd be pretty much non-existant (Hello Sidings!). Part of the fun of this hobby though is learning about how and why the prototype does what it does.....
Same with scenery. Veg is hard to do right, but I think alot of what people miss when they start out when they're trying for prototype is just how varied the Trackside Veg is, and just how much of it there is..... Its literally EVERYWHERE, and it comes in ALL sizes. A few bushes or trees will always look like something from toy town. But at the end of the day, thats ok. Trainz was built originally to be able to cater to people that wanted to make a virtual model railroad and its very good at that.
Also on a note about the veg, honestly, don't get too serious about it if you're worried about your computers specs. To really Run high quality Veg routes you need mid-high end computer with a decent Graphics card, and thats before you even contemplate Speed Trees. You can get away with high poly buildings, Rolling Stock, and even Track with mid range comps and mid range graphics cards. But throw in enough High Poly Veg with Splines and what not, and it becomes a bear real quick. Not saying it can't be done, but it generally takes more pro-active resource management and patience.
Textures are a bear in and of themselves, only because there are so many of them. I use a wide variety of them at this point. Some SAP stuff, some Ultra Textures, some TrainzItalia stuff.... But I'm going for Pacific Northwest Wet/Lowlands. It really comes down to downloading as many as you can find and trolling through them occasionally and experimenting with what you have. Then 2 months later after you paint up 10 baseboards finding a better Dirt or Concrete Texture.... Luckily Replace Asset works with Textures as well, lol. Copy and Paste can be a huge help too. One of the things to remember though: When you find a good texture you like, Paint some somewhere it'll be easy to find and grab, and then you don't have to keep a list of it or anything. Same goes for Objects you like, or Splines. Drop one or two somewhere easy to find. The route Im working on atm is big enough I have a few "Palette"s around I grab from.
Another recommendation for Steam Locomotives, particularly if you don't mind paying a reasonable price (FAR More reasonable then what you'd be Dropping for N-Scale stuff):
http://www.kltrainz.com/
If you've been in the South East for awhile, you'll like alot of their more recent stuff.
Good Luck,
Falcus