We've hit some stumbling blocks in the drawing department at our home recently. My son will paint or do sculptures, but drawing (or especially writing) are very frustrating for him. He wants them to look "right" and when they don't he doesn't see the point, really.
When we used up the printed sheets, I made him some more with washi tape. You just need wheels (which, theoretically, your children could draw) and then you make the cars with long pieces of tape. He added some in for the track, too. We had a lot of fun with these and at the end, we also got one completely free-form train (bottom), which was a big step forward.