Railworks America

I'm fairly new to Railworks so I apologize if this has been addressed before. What I want to know is, how do I replace the default rolling stock in the scenarios with more realistic looking looking freight cars that I see in people's videos? For example, the double stacks are all plain. Is there an easy way to do this? Do I have to go into the route editor and replace each and every car? Any help as far as pointing me to good websites to download realistic freight (graffiti, weathered, etc) and how to install them would be greatly appreciated.

One of the easier way to swap out locos or stock on a scenario, and without losing the scenario's existing instructions, is by using Mike's RW_Tools Utility to do this:
http://www.rstools.info/rw_tools.html
Read the documents that come with it and on Mike's site. It takes a bit of getting used to learn how to use it, but it's a great utility that has a lot of nice features it can help one do some things much quicker and easier.

I'd suggest to make a Cloned version of the scenario first you want to edit so as not to risk messing-up the original, and also if updates are made to RailWorks or you had to do a Verify Integrity then it won't affect the edits made on the cloned version.