It has taken me a long time to get this far.  I am presenting evidence of failure of 3D Contact at face pressures found in real world engineering; wheel/rail contact in my case, and I would expect also in gear tooth contact.

I plot the wheel/rail contact force as the train rolls around the track.  Most times the force abruptly zeros before the timeline completes.  There is no correlation with train geometry or speed, or track geometry.

The screen shots show camera views of the wheels on the rails milliseconds before and after the abrupt zeroing of contact force.

I suggest that contact fails instantly on all wheels simultaneously, and gravity sends the train plunging toward the centre of the Earth.  The train's hoizontal velocity through space continues up to the end of the timeline.  I have not managed to capture all this action on one video.

Here are my settings.

Any suggestions?