I concur with Blackhawk. My advice to you is to become familiar with Trainmaster. It is a lot harder to play than 1.05 or 1.06. It's designed for bigger and more complicated scenarios.
Then maybe i would suggest looking into creating a TM scenario. If in making this scenario you want to make a custom pak then I will assist you.
I do not recommend making a new locomotive at this point unless it is important to your scenario, and then I can help you.
Making stuff in TM takes a long time, and sometimes you need to put it away and come back to it. I have been working on a scenario for a year now. Eventually I will finish it and then probably make some new locomotives, and other things. But I am taking long approach...you should have the same patience or you might find yourself overwhelmed quickly and lose interest. It's happened a lot before once people learn how much work is involved. For example, when I was making the dorchester 0-4-0 it took me a week of constant work to put the second set of driving wheels. I am still not completely sastisfied with the cowboy engines, so that's why they are only available through attachments to posts.
Think about what is most important to you, do that project, and go from there.