Yes. I believe Dirt Rally WRC23 by EA/Codemasters will have a feature to build your own rally car. So it is possible. I even had this idea too.... very good!!!
'Make my own train' as in making your consist by coupling the stock of your choice OR modeling the stock on a CAD software and adding physics with simugraph ?!
That's actually a great idea, but it would be an entirely different game. It won't fit in the TSWs environment...
I was thinking about stuff like that, however limited to like changing the specs! For example, you can change a UP SD70ACe into a NS speced one, or a CSX SD40-2 with lower ditchlights.
Doing something simple like changing headlight orientation, or switching between different motors available for the irl models could work