Welcome to the forums and to Trainz a new world which will keep you occupied for a lifetime!
The things you are requesting here are all possible in TS12.
You can clone the route by opening the route for editor in Surveyor, then choosing to save it. You will be prompted to save a copy because you can't overwrite the built-in one. With your own route, you can do whatever you wish including adding traffic, adding more baseboards, merging with other routes and many, many other things.
To add traffic, edit the route in Surveyor then pick Main menu Edit route. (Don't ask why it's called this). Here you can change the region to something which will have traffic such as United States. There are even others available on the Download Station (DLS) which will add different vehicles, and you can modify these to create your own flavor. This aspect is a bit advanced for now so we'll leave that for another topic.
Content, as has been mentioned, is available on the Download Station (DLS), or from third-parties such as Jointed Rail
www.jointedrail.com. If you are going to do a lot of downloading, and this seems to be the case, we highly recommend a First Class Ticket. They cost about $25 to $30 USD for a year. (The price shown on the Simulator Central website (N3V's store), are in Australian dollars which are a lot higher). This will give you unlimited download capabilities, unlike the default 100MB per day without one on a very slow connection.
When setting up running sessions, you can filter the content to show only US and or Canadian content. This will eliminate the European stuff you don't want. I recommend looking at some of the European buildings and stuff in case you decide to build routes because much of it is applicable. And for non-European stuff, look at the stuff from Australia and New Zealand. Where the content creators used those places as the region, however, the buildings are applicable for here.
In the Edit/Create session, you can setup and save multiple AI drivers, add extra drivers, and add all kinds of destinations rules for them to go to. This will give you many, drivers to fill your routes. There are even ways of saving a library, called the Schedule Library, to save a string of driver commands so you don't have to re-enter these over and over for each driver. There are also portals which are placed in Surveyor, but setup in the similar-looking Session editor, that will spit out full AI-controlled trains which you have assigned driver commands to. These will appear at a set time, you specify, and will create extra traffic on the route.
My favorite thing with this is to let the AI run lots of traffic while I take over a train of my own. I usually switch a yard or run locals and commuter trains while the AI are driving. Periodically, as when I switch the yards, I will take over an AI-controlled train, uncouple the engines and send them to the engine house while I switch out the cars. When that's all finished, I recouple the AI-driven locos and have the train continue on its way.
So yes in so many words, what you have asked can be done and much, much more. If you noticed many of us have a join date on the forums of Nov 2006. This date is actually wrong and many of us here have been here since 2001. I started in December 2003 and haven't stopped using the program and its iterations since. As I said, you've acquired an addicting and absorbing program which will hopefully give you many years of enjoyment.
In closing, ensure you are properly registered. You need to enter in the serial number in your My Trainz account area as well as entering in the serial number when first installing. Once this is done, the program should appear on your station line, and if not you will need to submit a ticket. By registering, you will be able to download the content and have access to other aspects of the forums.
John