I worked on MT3 in two parts today. Right around 2 hours total, I think, not including letting glue dry (during which I ate lunch and had a nice nap). I found a pretty good technique for keeping the large marble in place: using a twist tie looped around the track seems to do the trick. If I had taken the time to figure that out before I had the guy push the big marble up the M Splitter, he would have been able to walk up on both rails. As it is, he had one foot down and one foot up, lodged betwen the rail and the marble to keep it from rolling.
Now, as of the last frame in the video below, the marble is being kept in place with a twist tie. I have marked some graduated lines on Dragonframe to show how far the marble should move between frames. Should be pretty easy to do the next frames, and if I can get the timing right, I will have the marble push the white bar into the back wall, just as a visual gag and exposition of just how massive the big marble is.
Second of two live streams from today: https://www.youtube.com/watch?v=3uneHQljsOc
Since June 2017, I have spent 4.8+ days on Marble Track 3.