As far as cad work on a locomotive, for the most part, each piece is either round or square. This makes most of the drawing relatively simple. Also, when you build your model, you can decide exactly how much detail you want to put into both the entire model, and each individual piece. I have seen store bought locos where the driver spokes are squared and flat. This is not the case for 99% (a guess) of prototype locomotives. Yet it passes for an off the shelf item. One thing that is hard to get over is, if you have a scratch built or kit built model, you will know every single flaw or cut corner for that specific model. But most people will never see them. Build it to your skill level, and enjoy it. Every modeler has built a model that they probably wont run on their final layout. You learn with each build and improve on the next...
The one serious thing that I would recommend is this; When you build your model, don't just glue and solder everything together. If it can be bolted on, or made to be removed later, do it. We have had our two finished locomotives torn almost completely back apart to fix things we didn't like about the original build...
Casey