While I love your aspirations.
Before making an 80 passenger car, and a complex locomotive, why not make a box car or something more simple.
Then you can learn about trucks, equalization, bolsters, track and wheel gauge, etc.
All fundamentals you will need for more complex things. Say you wanted to be a Formula 1 driver, your first race would not be at Monza or Monaco, but in a go cart on a local track.
Similarly, trying to handle complex things without even the fundamentals seems pretty hard to do, very few people, even mechanical engineers have started at 1,000 miles an hour!
Make something. Learn the basics of 3d printing. Make a simple box car, you will have enough challenges with that. Setting your sights too high initially will probably end in frustration.