I first printed out a couple of train silhouettes from the internet and since I didn't like the direction they were going in, I used their negative. Then I taped down a huge piece of wrapping paper on our dining room table. I knew that I was going to need to establish some markers for the size the train will end up being relative to the printed train. I decided to use salad plates as the train "wheels". I was then able to use the wheels as a basis for all of my other dimensions and only had to freehand a couple of parts. As you can see from the photo below, I used several things I had lying around...a level, a Harry Potter book, a boot box lid, etc.
Once I had it all drawn out in pencil, I went back over it in black marker and then cut the stencil out. By this time it was late at night and I knew the project could be postponed till the next day.

I then taped the stencil up on the wall and traced around it in pencil. Then I removed the stencil and started filling the shape in with chalkboard paint. I ended up doing 2 coats of the paint with a large brush and then going back around the edges with a smaller brush so that I would have as even of lines as I could muster.
Voila! B hasn't seen it yet but he did carry the printed train pages around the house all weekend after he saw them. He kept saying they were his choo choo trains.