Get a clear picture of what you want to make, then break it down into simple shapes, think rectangles, squares, triangles, circles. Then start building from there. I made an iMac computer one day out of boredom. Then, sew it all together and stuff it.
My suggestion is to use something like red heart super saver becaused its washable, and use a smaller than recommended needle size with single crochets, to keep the fabric tight.
If you want, I could try to make one for you and write the pattern down or something...
I'm thinking a nice round tube for the body of the engine, a square for the rest, circles for the wheels... etc.