Hi Mike ,
Actually I go into sidings and take measurements off cars and Photos .
A lot of dimensional data is right on the cars . I am not a exacto, perfecto on every last thing . I do get the weathering and graffiti as close as possible .
Scratch building is an acquired skill. The more you do it the better the results and have patience . For example to make a master for molding on say a Hy-cube box car , it may take 100 hrs to make the master . Tough sledding . But to do just one , old model magazines have many drawings and blue prints . I use these to.
Here is a car I am in process of making .