We bought some new roller coaster cars on BrickLink because a couple of the wheels/axles were just slightly worn and the cars wouldn't make it all the way down. Now it works great!

I was surprised at how it can still be a really creative set for the kids, they don't try to modify the track (that'd be too hard to make it work), but they have fun building it into a whole amusement park with other Lego sets and bricks.