Trouble closing a Nurb tube.
 

Hello everyone,

I'm making a Marble Machine (like the one in the movie Fracture).
For the modelling of the track I first make 3 curves of the form that I want the track to have. Next I make a Nurbs circle, I duplicate that circle so that every curve has 1 circle and then I extrude the circle along the path to get a polygon tube.

Next I make the ball an active and the track a passive rigid object. Everything works fine except the ball get's stuck in the track (a rigid body penetration error confirms this) and won't move. I tried upping the solver step size up to the point where Maya froze when I hit play but no setting worked as far as I could tell.

So next I tried extruding the circles a nurb tubes but the problem with those is that I can't turn an unclosed tube into a passive rigid body. I tried selecting the isoparm edge of 1 end and the using Surfaces -> Edit NURBs -> Close/Open Surface, but this doesn't give the desired result no matter what setting I use.

So my questions are (to sum it up):

Is a nurb track indeed better then a polygon for me as far as the collisions go? Or should I try a different approach?
How do I close a nurb tube? (something like the polygon "fill hole" option would be ideal)
Why can an open polygon tube be a rigid body? While a nurb tube cannot?

I hope the render gives a good enoug view of the situation and the open tubes.

Solved it, the problem was that I had to delete the history of the extruded nurb tube. After that everything worked out fine.

The ball doesn't sink in the nurb rigid.